Identity API: Add chrome.identity.onSignInChanged routing and IDL

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.
 
 {
   "activeTab": {
     "channel": "stable",
@@ skipping 342 matching lines @@
     ]
   },
   "history": {
     "channel": "stable",
     "extension_types": ["extension", "legacy_packaged_app"]
   },
   "identity": {
     "channel": "stable",
     "extension_types": ["extension", "platform_app"]
   },
+  "identity.email": {
+    "channel": "dev",
+    "extension_types": ["extension", "platform_app"]
+  },
   "identityPrivate": {
     "channel": "stable",
     "extension_types": [
       "legacy_packaged_app"
     ],
     "location": "component"
   },
   "idltest": {
     "channel": "trunk",
     "extension_types": ["extension"]
@@ skipping 438 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": [
       "mfffpogegjflfpflabcdkioaeobkgjik"  // GAIA Component Extension
     ]
   }]
 }