sync: remove use of protobuf extensions in protocol to reduce static init overhead.

chrome/browser/sync/protocol/app_notification_specifics.proto

 // 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.
 //
 // Sync protocol datatype extension for app notifications.
 
 // 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";
-
 // Properties of an app notification.
 
 // An App Notification, to be delivered from Chrome Apps to the
 // Chrome browser through the Notification API.
 message AppNotification {
   // Globally unique id. This is more robust for uniquely identifying each
   // notification and hence gives us flexibility in the future. In absence
   // of this, unique id would be (app_id, creation_timestamp_ms). But that
   // relies on creation_timestamp_ms being high resolution and is not
   // globally unique - only unique for a given user.
   optional string guid = 1;
   // Metadata, not shown directly to the user.
   // The unique App Id, as created by the webstore and used to 
   // delegate messages to the applications. This is defined as 32 characters
   optional string app_id = 2;
   // Timestamp when the message was created in milliseconds.
   // This is seperate from ctime as this is only set by the application.
   optional int64 creation_timestamp_ms = 3;
 
   // Payload - these fields are visible to the user content is defined by the
   // app. The fields are described in:
   // chrome/browser/extensions/app_notification.h
   optional string title = 4;
   optional string body_text = 5;
   optional string link_url = 6;
   optional string link_text = 7;
 }
 
-extend EntitySpecifics {
-  optional AppNotification app_notification = 45184;
-}