Add a Restore() method to ValueStore and make StorageAPI use it

chrome/browser/extensions/api/storage/settings_storage_factory.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_STORAGE_FACTORY_H_
 #define CHROME_BROWSER_EXTENSIONS_API_STORAGE_SETTINGS_STORAGE_FACTORY_H_
 
 #include <string>
 
 #include "base/files/file_path.h"
 #include "base/memory/ref_counted.h"
 
 class ValueStore;
 
 namespace extensions {
 
 // Factory for creating SettingStorage instances.
 //
 // Refcouted because it's just too messy to distribute these objects between
 // SettingsBackend instances any other way.
 class SettingsStorageFactory
     : public base::RefCountedThreadSafe<SettingsStorageFactory> {
  public:
   // Creates a new ValueStore area for an extension under |base_path|.
   // Return NULL to indicate failure.  Must be called on the FILE thread.
   virtual ValueStore* Create(const base::FilePath& base_path,
                              const std::string& extension_id) = 0;
 
+  // Deletes the database for the extension, if one exists.
+  // Note: it is important to delete references to the database if any are
+  // held, because ValueStores will create themselves if there is no file.
+  virtual void DeleteDatabaseIfExists(const base::FilePath& base_path,
+                                      const std::string& extension_id) = 0;
+
  protected:
   friend class base::RefCountedThreadSafe<SettingsStorageFactory>;
   virtual ~SettingsStorageFactory() {}
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_API_STORAGE_SETTINGS_STORAGE_FACTORY_H_