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

chrome/browser/sync/protocol/extension_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 extensions.
 
 // 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 extension sync objects.
 //
 // Merge policy: the settings for the higher version number win; in
 // the case of a tie, server wins.
 message ExtensionSpecifics {
   // Globally unique id for this extension.
   optional string id = 1;
   // The known installed version.
   optional string version = 2;
 
   // Auto-update URL to use for this extension.  May be blank, in
   // which case the default one (i.e., the one for the Chrome
   // Extensions Gallery) is used.
   optional string update_url = 3;
   // Whether or not this extension is enabled.
   optional bool enabled = 4;
   // Whether or not this extension is enabled in incognito mode.
   optional bool incognito_enabled = 5;
 
   // The name of the extension.  Used only for debugging.
   optional string name = 6;
 }
 
-extend EntitySpecifics {
-  optional ExtensionSpecifics extension = 48119;
-}