Add ExtensionsApiClient interface, use it in the storage API SettingsFrontend

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

 // 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.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_API_STORAGE_SETTINGS_BACKEND_H_
 #define CHROME_BROWSER_EXTENSIONS_API_STORAGE_SETTINGS_BACKEND_H_
 
 #include <string>
 
 #include "base/files/file_path.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "chrome/browser/extensions/api/storage/settings_storage_quota_enforcer.h"
 
 namespace syncer {
 class SyncableService;
 }
 
 class ValueStore;
 
 namespace extensions {
 class SettingsStorageFactory;
 
+// Performs work for the storage API on the FILE thread.
 class SettingsBackend {
  public:
   SettingsBackend(const scoped_refptr<SettingsStorageFactory>& storage_factory,
                   const base::FilePath& base_path,
                   const SettingsStorageQuotaEnforcer::Limits& quota);
   virtual ~SettingsBackend();
 
   // Gets a weak reference to the storage area for |extension_id|.
   // Must be run on the FILE thread.
   virtual ValueStore* GetStorage(const std::string& extension_id) = 0;
@@ skipping 30 matching lines @@
 
   // Quota limits (see SettingsStorageQuotaEnforcer).
   const SettingsStorageQuotaEnforcer::Limits quota_;
 
   DISALLOW_COPY_AND_ASSIGN(SettingsBackend);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_API_STORAGE_SETTINGS_BACKEND_H_