isolate cast_unittests

testing/buildbot/chromium_trybot.json

 {
   "compile_targets": [
     "all"
   ],
   "gtest_tests": [
     {
       "test": "accessibility_unittests",
       "swarming": {
         "can_use_on_swarming_builders": true
       }
@@ skipping 33 matching lines @@
       }
     },
     {
       "test": "browser_tests",
       "swarming": {
         "can_use_on_swarming_builders": true,
         "shards": 8
       }
     },
     "cacheinvalidation_unittests",
-    "cast_unittests",
+    {
+      "test": "cast_unittests",
+      "swarming": {
+        "can_use_on_swarming_builders": true
+      }
+    },
     {
       "test": "cc_unittests",
       "swarming": {
         "can_use_on_swarming_builders": true
       }
     },
     "chromedriver_unittests",
     {
       "test": "chromevox_tests",
       "chromium_configs": [
@@ skipping 233 matching lines @@
   "non_filter_tests_builders": [
   ],
 
   "filter_compile_builders": [
     "win_chromium_compile_dbg",
     "win_chromium_rel_swarming",
     "win_chromium_x64_rel_swarming",
     "win8_chromium_rel"
   ]
 }