[Sync] Stop accessing BrowserContextKeyedServiceFactory on non-UI thread.

chrome/browser/extensions/api/storage/sync_value_store_cache.h

 // 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.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_API_STORAGE_SYNC_VALUE_STORE_CACHE_H_
 #define CHROME_BROWSER_EXTENSIONS_API_STORAGE_SYNC_VALUE_STORE_CACHE_H_
 
 #include <memory>
+#include <string>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/weak_ptr.h"
 #include "components/sync/model/syncable_service.h"
 #include "extensions/browser/api/storage/settings_observer.h"
 #include "extensions/browser/api/storage/value_store_cache.h"
 
 namespace base {
 class FilePath;
 }
 
 namespace syncer {
 class SyncableService;
 }
 
 namespace extensions {
 
 class SyncStorageBackend;
 class ValueStoreFactory;
 
 // ValueStoreCache for the SYNC namespace. It owns a backend for apps and
 // another for extensions. Each backend takes care of persistence and syncing.
 class SyncValueStoreCache : public ValueStoreCache {
  public:
   SyncValueStoreCache(const scoped_refptr<ValueStoreFactory>& factory,
                       const scoped_refptr<SettingsObserverList>& observers,
                       const base::FilePath& profile_path);
   ~SyncValueStoreCache() override;
 
+  base::WeakPtr<SyncValueStoreCache> AsWeakPtr();
+
   syncer::SyncableService* GetSyncableService(syncer::ModelType type) const;
 
   // ValueStoreCache implementation:
   void RunWithValueStoreForExtension(
       const StorageCallback& callback,
       scoped_refptr<const Extension> extension) override;
   void DeleteStorageSoon(const std::string& extension_id) override;
 
  private:
   void InitOnFileThread(const scoped_refptr<ValueStoreFactory>& factory,
                         const scoped_refptr<SettingsObserverList>& observers,
                         const base::FilePath& profile_path);
 
   bool initialized_;
   std::unique_ptr<SyncStorageBackend> app_backend_;
   std::unique_ptr<SyncStorageBackend> extension_backend_;
+  base::WeakPtrFactory<SyncValueStoreCache> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncValueStoreCache);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_API_STORAGE_SYNC_VALUE_STORE_CACHE_H_