Support setting 'better' and using it in expectations.

tools/perf_expectations/perf_expectations.json

 {"linux-release-64/sizes/chrome-bss/bss": {"reva": 91085, "revb": 91326, "type": "absolute", "improve": 565495, "regress": 625223, "sha1": "eac0d7ac"},
  "linux-release-64/sizes/chrome-data/data": {"reva": 91119, "revb": 91326, "type": "absolute", "improve": 2309480, "regress": 2557742, "sha1": "fd84a9d2"},
  "linux-release-64/sizes/chrome-si/initializers": {"reva": 99922, "revb": 100028, "type": "absolute", "improve": 292, "regress": 326, "sha1": "07984766"},
  "linux-release-64/sizes/chrome-text/text": {"reva": 91010, "revb": 91326, "type": "absolute", "improve": 58265824, "regress": 64542416, "sha1": "e45ea3d9"},
  "linux-release-64/sizes/chrome.pak/chrome.pak": {"reva": 93584, "revb": 93660, "type": "absolute", "improve": 2922846, "regress": 3230516, "sha1": "a32b48c6"},
  "linux-release-64/sizes/chrome/chrome": {"reva": 99734, "revb": 100028, "type": "absolute", "improve": 78759552, "regress": 87104926, "sha1": "48bce0e1"},
  "linux-release-hardy/dhtml/commit_charge/cc": {"reva": 63231, "revb": 64325, "improve": -6947, "regress": 25528, "sha1": "0e22e67a"},
  "linux-release-hardy/dhtml/processes/proc_": {"improve": 0, "regress": 0, "sha1": "696fcb0f"},
  "linux-release-hardy/dhtml/times/t": {"reva": 99393, "revb": 100034, "improve": -68, "regress": 21, "sha1": "9c13cabb"},
  "linux-release-hardy/dhtml/total_byte_b/IO_b": {"reva": 85983, "revb": 88209, "improve": 4132, "regress": 6013, "sha1": "b7cb3bfb"},
@@ skipping 29 matching lines @@
  "linux-release-lowmem/startup/warm/t": {"reva": 81070, "revb": 81884, "improve": -117, "regress": 181, "sha1": "c4652b14"},
  "linux-release-lowmem/sunspider/total/t": {"reva": 81217, "revb": 81884, "improve": -645, "regress": 722, "sha1": "35f98911"},
  "linux-release-lowmem/v8_benchmark/score/score": {"reva": 81069, "revb": 81884, "improve": 1196, "regress": -2933, "sha1": "9e618ede"},
  "linux-release/dhtml/times/t": {"reva": 99393, "revb": 100034, "improve": -21, "regress": 145, "sha1": "6cb344a1"},
  "linux-release/intl1/times/t": {"reva": 99393, "revb": 100034, "improve": 8, "regress": 350, "sha1": "e12bff42"},
  "linux-release/intl2/times/t": {"reva": 99393, "revb": 100034, "improve": -46, "regress": 117, "sha1": "99a6bf16"},
  "linux-release/morejs/times/t": {"reva": 99393, "revb": 100034, "improve": -46, "regress": 63, "sha1": "1209e4ac"},
  "linux-release/moz/times/t": {"reva": 99393, "revb": 100034, "improve": -40, "regress": 172, "sha1": "2cf4039f"},
  "linux-release/sizes/chrome-bss/bss": {"reva": 91085, "revb": 91326, "type": "absolute", "improve": 328321, "regress": 362950, "sha1": "6718b4cc"},
  "linux-release/sizes/chrome-data/data": {"reva": 98413, "revb": 98423, "type": "absolute", "improve": 1199515, "regress": 1325781, "sha1": "e8545530"},
- "linux-release/sizes/chrome-si/initializers": {"reva": 101631, "revb": 101632, "type": "absolute", "improve": 292, "regress": 295, "tolerance": 0, "sha1": "fb861f9b"},
+ "linux-release/sizes/chrome-si/initializers": {"reva": 101631, "revb": 101632, "type": "absolute", "better": "lower", "improve": 295, "regress": 295, "tolerance": 0, "sha1": "b3f205e4"},
  "linux-release/sizes/chrome-text/text": {"reva": 91010, "revb": 91326, "type": "absolute", "improve": 50924512, "regress": 56409963, "sha1": "c6eabdc8"},
  "linux-release/sizes/chrome.pak/chrome.pak": {"reva": 93584, "revb": 93660, "type": "absolute", "improve": 2922846, "regress": 3230516, "sha1": "71901043"},
  "linux-release/sizes/chrome/chrome": {"reva": 99734, "revb": 100028, "type": "absolute", "improve": 68631100, "regress": 75912682, "sha1": "62b2e7be"},
  "linux-release/startup/times/t": {"improve": -15, "regress": 15, "sha1": "c8f06f81"},
  "mac-release-10.5/dhtml/times/t": {"improve": -90, "regress": 90, "sha1": "63a69fd0"},
  "mac-release-10.5/intl1/times/t": {"reva": 80750, "revb": 81318, "improve": -340, "regress": 120, "sha1": "03456e1f"},
  "mac-release-10.5/intl2/commit_charge/cc": {"reva": 79662, "revb": 81211, "improve": -46238, "regress": 55911, "sha1": "4c75e106"},
  "mac-release-10.5/intl2/processes/proc_": {"improve": 0, "regress": 0, "sha1": "a1a53594"},
  "mac-release-10.5/intl2/times/t": {"reva": 80750, "revb": 81208, "improve": -1772, "regress": 1586, "sha1": "3b491046"},
  "mac-release-10.5/intl2/vm_rss_final_b/vm_rss_f_b": {"reva": 99507, "revb": 100009, "improve": -7881, "regress": 7858, "sha1": "db007fa1"},
@@ skipping 49 matching lines @@
  "xp-release-single-core/intl2/times/t": {"reva": 98602, "revb": 99994, "improve": -195, "regress": 1640, "sha1": "3ceca048"},
  "xp-release-single-core/morejs/times/t": {"reva": 98602, "revb": 99994, "improve": -238, "regress": 221, "sha1": "6d1b1e56"},
  "xp-release-single-core/moz/times/t": {"reva": 75758, "revb": 76221, "improve": -1705, "regress": 3304, "sha1": "b615a416"},
  "xp-release-single-core/startup/times/t": {"improve": -9, "regress": 9, "sha1": "b1bf03da"},
  "xp-release/sizes/chrome.dll/chrome.dll": {"reva": 100820, "revb": 100961, "type": "absolute", "improve": 35191526, "regress": 38926004, "sha1": "94524a77"},
  "xp-release/sizes/chrome.exe/chrome.exe": {"reva": 81814, "revb": 82805, "type": "absolute", "improve": 1068134, "regress": 1180570, "sha1": "e56118d2"},
  "xp-release/sizes/mini_installer.exe/mini_installer.exe": {"reva": 84829, "revb": 85049, "type": "absolute", "improve": 18061977, "regress": 20016999, "sha1": "aca62812"},
  "xp-release/sizes/setup.exe/setup.exe": {"reva": 81852, "revb": 82805, "type": "absolute", "improve": 1138662, "regress": 1258522, "sha1": "f1de9ae7"},
  "load": true
 }