Enable post-compile test isolation for all CI builders.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "client.skia",
       "--builder",
       "Mac Builder-Trybot",
@@ skipping 81 matching lines @@
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
     "env": {
-      "GYP_DEFINES": "clang=1 component=static_library gomadir='[BUILD]/goma' target_arch=ia32 test_isolation_mode=archive test_isolation_outdir=https://isolateserver.appspot.com use_goma=1"
+      "GYP_DEFINES": "clang=1 component=static_library gomadir='[BUILD]/goma' target_arch=ia32 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.mac.json",
       "/path/to/tmp/json"
@@ skipping 67 matching lines @@
       "--build_revision",
       "143121",
       "--factory-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"buildername\": \"Mac Builder-Trybot\", \"buildnumber\": 571, \"mastername\": \"client.skia\", \"parent_buildername\": null, \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
       "--build-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"buildername\": \"Mac Builder-Trybot\", \"buildnumber\": 571, \"mastername\": \"client.skia\", \"parent_buildername\": null, \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}"
     ],
     "name": "package build"
   }
 ]