[Media Router] Convert to use typemaps for media_router.mojom.

chrome/browser/media/router/media_source.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_source.h"
 
 #include <string>
 
 #include "chrome/browser/media/router/media_source_helper.h"
 
@@ skipping 19 matching lines @@
 }
 
 bool MediaSource::operator==(const MediaSource& other) const {
   return id_ == other.id();
 }
 
 std::string MediaSource::ToString() const {
   return "MediaSource[" + id_ + "]";
 }
 
+MediaSource::MediaSource() {}
+
 }  // namespace media_router