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

chrome/browser/sync/protocol/preference_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 preferences.
 
 // 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 preference sync objects.
 message PreferenceSpecifics {
   optional string name = 1;
   optional string value = 2;
 }
 
-extend EntitySpecifics {
-  optional PreferenceSpecifics preference = 37702;
-}