[Media Router] Convert MediaRouter to use GURL for presentation URLs.

chrome/browser/media/router/media_source.h

Index: chrome/browser/media/router/media_source.h
diff --git a/chrome/browser/media/router/media_source.h b/chrome/browser/media/router/media_source.h
index 1f3abac3721c4033af56e01474435f28b053beb1..279edf0c49ff3c23dd29763ea4e5395cf1c88e03 100644
--- a/chrome/browser/media/router/media_source.h
+++ b/chrome/browser/media/router/media_source.h
@@ -12,14 +12,19 @@
 
 #include "base/hash.h"
 
-namespace media_router {
+class GURL;
 
+// TODO(mfoltz): Right now this is a wrapper for std::string.  Factor methods
+// from media_source_helper here so this object becomes useful; and don't just
+// pass it around by Id.
+namespace media_router {
 
 class MediaSource {
  public:
   using Id = std::string;
 
   explicit MediaSource(const MediaSource::Id& id);
+  explicit MediaSource(const GURL& presentation_url);
   ~MediaSource();
 
   // Gets the ID of the media source.