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

chrome/browser/sync/protocol/extension_setting_specifics.proto

 // Copyright (c) 2011 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 an extension setting.
 
 // 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 setting sync objects.
 message ExtensionSettingSpecifics {
   // Id of the extension the setting is for.
   optional string extension_id = 1;
 
   // Setting key.
   optional string key = 2;
 
   // Setting value serialized as JSON.
   optional string value = 3;
 }
 
-extend EntitySpecifics {
-  optional ExtensionSettingSpecifics extension_setting = 96159;
-}