[Media Router] Design MediaRouter interface with stub implementation.

chrome/browser/media/router/media_route.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_route.h"
 
 #include "base/logging.h"
 #include "chrome/browser/media/router/media_source.h"
 
 namespace media_router {
 
 MediaRoute::MediaRoute(const MediaRouteId& media_route_id,
                        const MediaSource& media_source,
                        const MediaSink& media_sink,
                        const std::string& description,
                        bool is_local)
     : media_route_id_(media_route_id),
       media_source_(media_source),
       media_sink_(media_sink),
       description_(description),
       is_local_(is_local),
       state_(MEDIA_ROUTE_STATE_NEW) {
 }
 
+MediaRoute::MediaRoute()

[mark a. foltz, 2015/03/27 21:21:57]

What is the use case for this?  There's no setters for the fields, so is it just
used as a Null Object?

http://en.wikipedia.org/wiki/Null_Object_pattern

It might be worth adding a brief comment to explain.

[imcheng, 2015/03/30 22:49:08]

Kevin suggested that instead of using a nullptr to represent a situation where a
route is absent (such as the error case of MediaRouteResponse). I added comments
in the header file.

+    : MediaRoute("", MediaSource(), MediaSink(), "", false) {
+}
+
 MediaRoute::~MediaRoute() {
 }
 
+bool MediaRoute::Empty() const {
+  return media_route_id_.empty();
+}
+
 bool MediaRoute::Equals(const MediaRoute& other) const {
   return media_route_id_ == other.media_route_id_;
 }
 
 }  // namespace media_router