[Media Router] Add experiment control logic.

chrome/browser/media/router/media_router_feature.cc

+// Copyright 2015 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 "chrome/browser/media/router/media_router_feature.h"
+
+#if !defined(OS_ANDROID)
+#include "extensions/common/feature_switch.h"
+#endif
+
+namespace media_router {
+
+bool MediaRouterEnabled() {
+#if defined(OS_ANDROID)
+  return true;
+#else
+  return extensions::FeatureSwitch::media_router()->IsEnabled();
+#endif  // defined(OS_ANDROID)
+}
+
+}  // namespace media_router