[Media Router] (device discovery) Create MediaSinkService interface

chrome/browser/media/router/media_sink_service.cc

+// Copyright 2017 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_sink_service.h"
+
+namespace media_router {
+
+MediaSinkService::MediaSinkService(
+    const OnSinksDiscoveredCallback& sinks_discovered_callback)
+    : sinks_discovered_callback_(sinks_discovered_callback) {}
+
+MediaSinkService::~MediaSinkService() = default;
+
+}  // namespace media_router