Add hook for extension to check sync status.

chrome/common/extensions/api/_permission_features.json

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // See c/c/e/features/* to understand this file, in particular feature.h,
 // simple_feature.h, and base_feature_provider.h.
 
 // To add a new whitelisted ID, SHA-1 it and force it to uppercase. In Bash:
 //
 // $ echo -n "aaaabbbbccccddddeeeeffffgggghhhh" | \
@@ skipping 649 matching lines @@
     "channel": "stable",
     "extension_types": [
       "extension", "legacy_packaged_app", "platform_app"
     ]
   },
   "preferencesPrivate":  {
     "channel": "dev",
     "extension_types": [
       "extension"
     ],
-    "location": "component"
+    "whitelist": [
+      "C41AD9DCD670210295614257EF8C9945AD68D86E",  // Google Now
+      "D5736E4B5CF695CB93A2FB57E4FDC6E5AFAB6FE2",  // http://crbug.com/312900
+      "D57DE394F36DC1C3220E7604C575D29C51A6C495"   // http://crbug.com/319444
+    ]
   },
   "principalsPrivate": {
     "channel": "stable",
     "extension_types": ["extension"],
     "whitelist": [
       "2779FA8B45841D61A37207CCFAC9CB393964FE5B"  // Login Proxy (prototype)
     ]
   },
   "privacy": {
     "channel": "stable",
@@ skipping 251 matching lines @@
     // Only allowed for whitelisted extensions until all the caveats are
     // addressed. Tracked in crbug/285151.
     "channel": "stable",
     "extension_types": ["extension"],
     "location": "component",
     "whitelist": [
       "D519188F86D9ACCEE0412007B227D9936EB9676B"  // GAIA Component Extension
     ]
   }]
 }