Add ExtensionsApiClient interface, use it in the storage API SettingsFrontend

extensions/browser/api/extensions_api_client.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 "extensions/browser/api/extensions_api_client.h"
+
+namespace extensions {
+namespace {
+
+ExtensionsApiClient* g_instance = NULL;
+
+}  // namespace
+
+ExtensionsApiClient::ExtensionsApiClient() {
+  g_instance = this;
+}
+
+ExtensionsApiClient::~ExtensionsApiClient() {
+  g_instance = NULL;
+}
+
+// static
+ExtensionsApiClient* ExtensionsApiClient::Get() { return g_instance; }
+
+void ExtensionsApiClient::AddAdditionalValueStoreCaches(
+    content::BrowserContext* context,
+    scoped_refptr<SettingsStorageFactory> factory,
+    scoped_refptr<ObserverListThreadSafe<SettingsObserver> > observers,
+    std::map<settings_namespace::Namespace, ValueStoreCache*>* caches) {}
+
+}  // namespace extensions