Add new flag to trace-wrappables bot

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[BUILDER_CACHE]/WebKit_Linux___TraceWrappables",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 449 matching lines @@
       "/path/to/tmp/json",
       "--test-results-server",
       "test-results.appspot.com",
       "--build-number",
       "571",
       "--builder-name",
       "WebKit Linux - TraceWrappables",
       "--step-name",
       "webkit_tests",
       "--additional-driver-flag",
-      "--enable-blink-features=TraceWrappables"
+      "--enable-blink-features=TraceWrappables",
+      "--additional-driver-flag",
+      "--js-flags=--incremental-marking-wrappers"
     ],
     "cwd": "[BUILDER_CACHE]/WebKit_Linux___TraceWrappables",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "webkit_tests",
     "~followup_annotations": [
       "@@@STEP_TEXT@<br/>Total executed: 9001<br/>@@@"
     ]
   },
@@ skipping 44 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@sys.exit(1)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]