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

chrome/browser/sync/protocol/search_engine_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 custom search engines.
 
 // 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 custom search engine sync objects.
 message SearchEngineSpecifics {
   // The description of the search engine.
   optional string short_name = 1;
   // The search engine keyword for omnibox access.
   optional string keyword = 2;
   // A URL to the favicon to show in the search engines options page.
   optional string favicon_url = 3;
   // The actual parameterized search engine query URL.
   optional string url = 4;
@@ skipping 25 matching lines @@
   // The parameterized URL that is used for Instant results.
   optional string instant_url = 15;
   // ID 16 reserved - previously used by |id|, now deprecated.
   // The last time this entry was modified by a user. A UTC timestamp with units
   // in microseconds.
   optional int64 last_modified = 17;
   // The primary identifier of this search engine entry for Sync.
   optional string sync_guid = 18;
 }
 
-extend EntitySpecifics {
-  optional SearchEngineSpecifics search_engine = 88610;
-}