Add media router common classes and unittests.

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 <string>
+
+namespace media_router {
+
+class MediaSource {
+ public:
+  explicit MediaSource(const std::string& id);
+  ~MediaSource();
+
+  // Gets the ID of the media source.
+  std::string id() const;
+
+ private:
+  std::string id_;
+};
+
+}  // namespace media_router
+
+#endif  // CHROME_BROWSER_MEDIA_ROUTER_MEDIA_SOURCE_H_