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_client = NULL;
+
+}  // namespace
+
+ExtensionsApiClient::ExtensionsApiClient() {}
+
+ExtensionsApiClient::~ExtensionsApiClient() {}
+
+void ExtensionsApiClient::AddAdditionalValueStoreCaches(
+    content::BrowserContext* context,
+    scoped_refptr<SettingsStorageFactory> factory,
+    scoped_refptr<ObserverListThreadSafe<SettingsObserver> > observers,
+    std::map<settings_namespace::Namespace, ValueStoreCache*>* caches) {}
+
+// static
+ExtensionsApiClient* ExtensionsApiClient::Get() { return g_client; }
+
+// static
+void ExtensionsApiClient::Set(ExtensionsApiClient* client) {
+  g_client = client;

[Devlin, 2014/03/03 17:07:59]

Should we ever be able to quietly reset a client?  That is, could we
DCHECK(!g_client) here?

[James Cook, 2014/03/03 19:53:09]

We used to have DCHECK(!g_client) in the other extension client interfaces but I
had to take them out because it's useful to reset them during tests.

[Devlin, 2014/03/03 20:25:27]

Ah, okay.

+}
+
+}  // namespace extensions