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

chrome/browser/sync/protocol/password_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 password data.
 
 // 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";
 
 // These are the properties that get serialized into the |encrypted| field of
 // PasswordSpecifics.
 message PasswordSpecificsData {
   optional int32 scheme = 1;
   optional string signon_realm = 2;
   optional string origin = 3;
   optional string action = 4;
   optional string username_element = 5;
   optional string username_value = 6;
   optional string password_element = 7;
   optional string password_value = 8;
   optional bool ssl_valid = 9;
   optional bool preferred = 10;
   optional int64 date_created = 11;
   optional bool blacklisted = 12;
 }
 
 // Properties of password sync objects. The actual password data is held in a
 // PasswordSpecificsData that is encrypted into |encrypted|.
 message PasswordSpecifics {
   optional EncryptedData encrypted = 1;
 }
 
-extend EntitySpecifics {
-  optional PasswordSpecifics password = 45873;
-}