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

chrome/browser/media/router/media_source.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_SOURCE_H_
 #define CHROME_BROWSER_MEDIA_ROUTER_MEDIA_SOURCE_H_
 
+#include <ostream>
 #include <string>
 
+#include "base/hash.h"
+
 namespace media_router {
 
+using MediaSourceId = std::string;
+
 class MediaSource {
  public:
-  explicit MediaSource(const std::string& id);
+  explicit MediaSource(const MediaSourceId& id);
+  MediaSource();
   ~MediaSource();
 
   // Gets the ID of the media source.
-  std::string id() const;
+  MediaSourceId id() const;
+
+  // Returns true if two MediaSource objects use the same media ID.
+  bool Equals(const MediaSource& other) const;
+
+  // Returns true if a MediaSource is empty or uninitialized.
+  bool Empty() const;
+
+  // Used for logging.
+  std::string ToString() const;
+
+  // Hash operator for hash containers.
+  struct Hash {
+    size_t operator()(const MediaSource& source) const {
+      return base::Hash(source.id());
+    }
+  };
 
  private:
-  std::string id_;
+  MediaSourceId id_;
 };
 
 }  // namespace media_router
 
 #endif  // CHROME_BROWSER_MEDIA_ROUTER_MEDIA_SOURCE_H_