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

chrome/common/extensions/api/_api_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.
 
 {
   "activityLogPrivate": {
     "dependencies": ["permission:activityLogPrivate"],
@@ skipping 255 matching lines @@
   },
   "i18n": {
     "channel": "stable",
     "extension_types": ["extension", "legacy_packaged_app", "platform_app"],
     "contexts": ["blessed_extension", "unblessed_extension", "content_script"]
   },
   "identity": {
     "dependencies": ["permission:identity"],
     "contexts": ["blessed_extension"]
   },
+  "identity.onSignInChanged": {
+    "channel": "dev",
+    "dependencies": ["permission:identity"],
+    "contexts": ["blessed_extension"]
+  },
   "identityPrivate": {
     "dependencies": ["permission:identityPrivate"],
     "contexts": ["blessed_extension"]
   },
   "idltest": {
     "dependencies": ["permission:idltest"],
     "contexts": ["blessed_extension"]
   },
   "idle": {
     "dependencies": ["permission:idle"],
@@ skipping 331 matching lines @@
   "webview": {
     "internal": true,
     "dependencies": ["permission:webview"],
     "contexts": ["blessed_extension", "unblessed_extension"]
   },
   "windows": {
     "dependencies": ["api:tabs"],
     "contexts": ["blessed_extension"]
   }
 }