Add oauth client id to AppSpecifics. We will use that when a user revokes notification permission.

chrome/browser/sync/protocol/app_specifics.proto

 // Copyright (c) 2010 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.
 //
 // Sync protocol datatype extension for apps.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 option retain_unknown_fields = true;
 
 package sync_pb;
 
 import "sync.proto";
 import "extension_specifics.proto";
 
 // Settings related to push notifications for apps.
 message AppNotificationSettings {
+  // DEPRECATED: Use oauth_client_id below.
   // Whether or not the user has setup notifications at least once.
   // The value for this field will start out false and will be set
   // to true when the user accepts receiving notifications for the
   // first time and then it will always remain true.
   optional bool initial_setup_done = 1;
 
   // Whether or not the user has disabled notifications.
   optional bool disabled = 2;
+
+  // OAuth2 client id to which the user granted the notification permission.
+  // This field will start out empty.
+  // It will be set when the user accepts receiving notifications.
+  // This field is used when the user revokes the notifications permission.
+  // Note that it is never cleared after it was set once. Hence, the presence
+  // of this field can be used to determine if the user has setup notifications
+  // at least once for the given app.
+  optional string oauth_client_id = 3;
 }
 
 // Properties of app sync objects.
 //
 // For now, an app is just an extension.  We keep the two data types
 // separate for future-proofing purposes.
 message AppSpecifics {
   // Extension data.
   optional ExtensionSpecifics extension = 1;
 
   // Notification settings.
   optional AppNotificationSettings notification_settings = 2; 
 }
 
 extend EntitySpecifics {
   optional AppSpecifics app = 48364;
 }