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

chrome/browser/media/router/media_sink.h

 // 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.
 
 #ifndef CHROME_BROWSER_MEDIA_ROUTER_MEDIA_SINK_H_
 #define CHROME_BROWSER_MEDIA_ROUTER_MEDIA_SINK_H_
 
 #include <string>
 
 #include "chrome/browser/media/router/media_route_id.h"
 
 namespace media_router {
 
 using MediaSinkId = std::string;
 
 // Represents a sink to which media can be routed.
 class MediaSink {
  public:
-  // |sink_id|: Unique identifier for the MediaSink.
-  // |name|: Descriptive name of the MediaSink.
   MediaSink(const MediaSinkId& sink_id,
             const std::string& name);
+
   ~MediaSink();
 
-  const MediaSinkId& sink_id() const { return sink_id_; }
+  const MediaSinkId& id() const { return sink_id_; }
   const std::string& name() const { return name_; }
 
   bool Equals(const MediaSink& other) const;
+  bool Empty() const;
 
  private:
-  const MediaSinkId sink_id_;
-  const std::string name_;
+  // Unique identifier for the MediaSink.
+  MediaSinkId sink_id_;
+  // Descriptive name of the MediaSink.
+  // Optional, can use an empty string if no sink name is available.
+  std::string name_;
 };
 
 }  // namespace media_router
 
 #endif  // CHROME_BROWSER_MEDIA_ROUTER_MEDIA_SINK_H_