[Extensions] Pull ExtensionId into its own file

chrome/browser/extensions/api/storage/sync_value_store_cache.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 "chrome/browser/extensions/api/storage/sync_value_store_cache.h"
 
 #include <stddef.h>
 
 #include "chrome/browser/extensions/api/storage/sync_storage_backend.h"
 #include "chrome/browser/sync/glue/sync_start_util.h"
 #include "content/public/browser/browser_thread.h"
 #include "extensions/browser/value_store/value_store_factory.h"
 #include "extensions/common/api/storage.h"
+#include "extensions/common/extension.h"
 
 using content::BrowserThread;
 
 namespace extensions {
 
 namespace {
 
 // Returns the quota limit for sync storage, taken from the schema in
 // extensions/common/api/storage.json.
 SettingsStorageQuotaEnforcer::Limits GetSyncQuotaLimits() {
@@ skipping 74 matching lines @@
   extension_backend_.reset(new SyncStorageBackend(
       factory,
       GetSyncQuotaLimits(),
       observers,
       syncer::EXTENSION_SETTINGS,
       sync_start_util::GetFlareForSyncableService(profile_path)));
   initialized_ = true;
 }
 
 }  // namespace extensions