[Media Router] Reland: MediaRouter interfaces with stub implementations.

chrome/browser/media/router/media_sinks_observer.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_sinks_observer.h"
+
+#include "base/logging.h"
+#include "chrome/browser/media/router/media_router.h"
+
+namespace media_router {
+
+MediaSinksObserver::MediaSinksObserver(MediaRouter* router,
+                                       const MediaSource& source)
+    : source_(source), router_(router) {
+  DCHECK(router_);
+  if (!router_->RegisterMediaSinksObserver(this))
+    LOG(ERROR) << "RegisterMediaSinksObserver failed: " << source.ToString();
+}
+
+MediaSinksObserver::~MediaSinksObserver() {
+  router_->UnregisterMediaSinksObserver(this);
+}
+
+}  // namespace media_router