Add media router common classes and unittests.

chrome/browser/media/router/media_route_id.h

Index: chrome/browser/media/router/media_route_id.h
diff --git a/chrome/browser/media/router/media_route_id.h b/chrome/browser/media/router/media_route_id.h
new file mode 100644
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+++ b/chrome/browser/media/router/media_route_id.h
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+// 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_ROUTE_ID_H_
+#define CHROME_BROWSER_MEDIA_ROUTER_MEDIA_ROUTE_ID_H_
+
+#include <string>
+
+namespace media_router {
+
+using MediaRouteId = std::string;
+
+}  // namespace media_router
+
+#endif  // CHROME_BROWSER_MEDIA_ROUTER_MEDIA_ROUTE_ID_H_