Split ExtensionSystem interface from ExtensionSystemImpl implementation, part 1.

extensions/browser/extension_system.cc

+// Copyright 2013 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/extension_system.h"
+
+#include "chrome/browser/extensions/extension_system_factory.h"
+
+namespace extensions {
+
+ExtensionSystem::ExtensionSystem() {
+}
+
+ExtensionSystem::~ExtensionSystem() {
+}
+
+// static
+ExtensionSystem* ExtensionSystem::Get(content::BrowserContext* context) {
+  return ExtensionSystemFactory::GetForBrowserContext(context);
+}
+
+}  // namespace extensions