Don't set fastbuild=1 for Mac ASan trybots.

scripts/slave/recipes/chromium.expected/dynamic_gtest_memory_mac64.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "chromium.memory",
       "--builder",
       "Mac ASan 64 Tests (1)",
@@ skipping 63 matching lines @@
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
     "env": {
-      "GYP_DEFINES": "asan=1 clang=1 component=static_library gomadir='[BUILD]/goma' host_arch=x64 target_arch=x64 test_isolation_mode=prepare use_goma=1"
+      "GYP_DEFINES": "asan=1 clang=1 component=static_library fastbuild=0 gomadir='[BUILD]/goma' host_arch=x64 target_arch=x64 test_isolation_mode=prepare use_goma=1"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]/src/testing/buildbot/chromium.memory.json",
       "/path/to/tmp/json"
@@ skipping 98 matching lines @@
       "--builder-name=Mac ASan 64 Tests (1)",
       "--slave-name=TestSlavename",
       "--build-number=571",
       "--enable-asan",
       "browser_tests",
       "--test-launcher-print-test-stdio=always"
     ],
     "name": "browser_tests"
   }
 ]