[GCM] Move registration info persistence from extension state store to GCM store

google_apis/gcm/engine/gcm_store.cc

 // Copyright 2014 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.
 
 #include "google_apis/gcm/engine/gcm_store.h"
 
 namespace gcm {
 
-GCMStore::SerialNumberMappings::SerialNumberMappings()
-    : next_serial_number(1LL) {
-}
-
-GCMStore::SerialNumberMappings::~SerialNumberMappings() {
-}
-
 GCMStore::LoadResult::LoadResult()
     : success(false),
       device_android_id(0),
       device_security_token(0) {
 }
 
 GCMStore::LoadResult::~LoadResult() {}
 
 GCMStore::GCMStore() {}
 
 GCMStore::~GCMStore() {}
 
 }  // namespace gcm