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>
+
+#include "chrome/browser/media/router/media_route_id.h"
+
+namespace media_router {
+
+using MediaSource = std::string;
+
+// Returns MediaSource URI depending on the type of source.
+MediaSource ForTabMediaSource(int tab_id);
+MediaSource ForDesktopMediaSource();
+MediaSource ForCastAppMediaSource(const std::string& app_id);
+MediaSource ForPresentationUrl(const std::string& presentation_url);
+
+bool IsMirroringMediaSource(const MediaSource& source);
+bool IsValidMediaSource(const MediaSource& source);
+
+}  // namespace media_router
+
+#endif  // CHROME_BROWSER_MEDIA_ROUTER_MEDIA_SOURCE_H_