Reland: Add ExtensionsApiClient interface, use it in the storage API SettingsFrontend

extensions/browser/api/extensions_api_client.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 EXTENSIONS_BROWSER_API_EXTENSIONS_API_CLIENT_H_
+#define EXTENSIONS_BROWSER_API_EXTENSIONS_API_CLIENT_H_
+
+#include <map>
+
+#include "base/memory/ref_counted.h"
+#include "extensions/browser/api/storage/settings_namespace.h"
+
+template <class T>
+class ObserverListThreadSafe;
+
+namespace content {
+class BrowserContext;
+}
+
+namespace extensions {
+
+class SettingsObserver;
+class SettingsStorageFactory;
+class ValueStoreCache;
+
+// Allows the embedder of the extensions module to customize its support for
+// API features. The embedder must create a single instance in the browser
+// process. Provides a default implementation that does nothing.
+class ExtensionsAPIClient {
+ public:
+  // Construction sets the single instance.
+  ExtensionsAPIClient();
+
+  // Destruction clears the single instance.
+  virtual ~ExtensionsAPIClient();
+
+  // Returns the single instance of |this|.
+  static ExtensionsAPIClient* Get();
+
+  // Storage API support.
+
+  // Add any additional value store caches (e.g. for chrome.storage.managed)
+  // to |caches|. By default adds nothing.
+  virtual void AddAdditionalValueStoreCaches(
+      content::BrowserContext* context,
+      const scoped_refptr<SettingsStorageFactory>& factory,
+      const scoped_refptr<ObserverListThreadSafe<SettingsObserver> >& observers,
+      std::map<settings_namespace::Namespace, ValueStoreCache*>* caches);
+
+  // NOTE: If this interface gains too many methods (perhaps more than 20) it
+  // should be split into one interface per API.
+};
+
+}  // namespace extensions
+
+#endif  // EXTENSIONS_BROWSER_API_EXTENSIONS_API_CLIENT_H_