Enable post-compile test isolation for all CI builders.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "client.skia",
       "--builder",
       "Linux Tests-Trybot",
@@ skipping 81 matching lines @@
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
     "env": {
-      "GYP_DEFINES": "component=static_library gomadir='[BUILD]/goma' target_arch=x64 test_isolation_mode=archive test_isolation_outdir=https://isolateserver.appspot.com use_goma=1"
+      "GYP_DEFINES": "component=static_library gomadir='[BUILD]/goma' 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.linux.json",
       "/path/to/tmp/json"
@@ skipping 246 matching lines @@
       "https://chromium-swarm.appspot.com",
       "--decorate",
       "--print-status-updates",
       "--shards",
       "1",
       "browser_tests/Ubuntu-12.04/ffffffffff/Linux Tests-Trybot/571"
     ],
     "name": "browser_tests"
   }
 ]