[Media Router] Design MediaRouter interface with stub implementation.

chrome/browser/media/router/media_sink.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_sink.h"
 
 namespace media_router {
 
 MediaSink::MediaSink(const MediaSinkId& sink_id,
                      const std::string& name)
-    : sink_id_(sink_id), name_(name) {
-}
+    : sink_id_(sink_id), name_(name) {}
 
-MediaSink::~MediaSink() {
-}
+MediaSink::MediaSink() : MediaSink("", "") {}
+
+MediaSink::~MediaSink() {}
 
 bool MediaSink::Equals(const MediaSink& other) const {
   return sink_id_ == other.sink_id_;
 }
 
+bool MediaSink::Empty() const {
+  return sink_id_.empty();
+}
+
 }  // namespace media_router