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

scripts/slave/recipes/libyuv/libyuv.expected/client_libyuv_mac_asan.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "config",
       "--spec",
       "cache_dir = None\nsolutions = [{'custom_vars': {'root_dir': 'src'}, 'deps_file': 'DEPS', 'managed': True, 'name': 'src', 'url': 'svn://svn-mirror.golo.chromium.org/libyuv/trunk'}]"
     ],
@@ skipping 27 matching lines @@
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
     "env": {
-      "GYP_DEFINES": "asan=1 clang=1 component=static_library gomadir='[BUILD]/goma' lsan=1 target_arch=ia32 use_goma=1"
+      "GYP_DEFINES": "asan=1 clang=1 component=static_library fastbuild=0 gomadir='[BUILD]/goma' lsan=1 target_arch=ia32 use_goma=1"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/compile.py",
       "--target",
       "Release",
@@ skipping 26 matching lines @@
       "--enable-asan",
       "--enable-lsan",
       "--lsan-suppressions-file=[SLAVE_BUILD]/src/tools/lsan/suppressions.txt",
       "libyuv_unittest",
       "--test-launcher-print-test-stdio=always",
       "--test-launcher-batch-limit=1"
     ],
     "name": "libyuv_unittest"
   }
 ]