[Media Router] Design MediaRouter interface with stub implementation.

chrome/browser/media/router/media_source.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_source.h"
 
 #include <string>
 
 namespace media_router {
 
-MediaSource::MediaSource(const std::string& source_id)
-    : id_(source_id) {
+MediaSource::MediaSource(const MediaSourceId& source_id) : id_(source_id) {
+}
+
+MediaSource::MediaSource() {
 }
 
 MediaSource::~MediaSource() {}
 
-std::string MediaSource::id() const {
+MediaSourceId MediaSource::id() const {
   return id_;
 }
 
+bool MediaSource::Equals(const MediaSource& other) const {
+  return id_ == other.id();
+}
+
+bool MediaSource::Empty() const {
+  return id_.empty();
+}
+
+std::string MediaSource::ToString() const {
+  return "MediaSource[" + id_ + "]";
+}
+
 }  // namespace media_router