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

chrome/browser/sync/protocol/nigori_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 nigori keys.
 
 // 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 "encryption.proto";
-import "sync.proto";
 
 message NigoriKey {
   optional string name = 1;
   optional bytes user_key = 2;
   optional bytes encryption_key = 3;
   optional bytes mac_key = 4;
 }
 
 message NigoriKeyBag {
   repeated NigoriKey key = 2;
@@ skipping 57 matching lines @@
 
   optional bool encrypt_extension_settings = 25;
   optional bool encrypt_app_notifications = 26;
   optional bool encrypt_app_settings = 27;
 
   // User device information. Contains information about each device that has a
   // sync-enabled Chrome browser connected to the user account.
   repeated DeviceInformation device_information = 28;
 }
 
-extend EntitySpecifics {
-  optional NigoriSpecifics nigori = 47745;
-}