[MediaRouter] Add implementation of PresentationServiceDelegate

chrome/browser/media/router/media_source_helper.h

Index: chrome/browser/media/router/media_source_helper.h
diff --git a/chrome/browser/media/router/media_source_helper.h b/chrome/browser/media/router/media_source_helper.h
index a46294f870f5ce46391b034cc181572bfc9eb42f..933827ec0b99c37e6f0345961db8dc8f364e0385 100644
--- a/chrome/browser/media/router/media_source_helper.h
+++ b/chrome/browser/media/router/media_source_helper.h
@@ -8,6 +8,7 @@
 #include <string>
 
 #include "chrome/browser/media/router/media_source.h"
+#include "chrome/browser/media/router/presentation_helper.h"
 
 namespace media_router {
 
@@ -35,6 +36,11 @@ bool IsMirroringMediaSource(const MediaSource& source);
 // Does not deeper protocol-level syntax checks.
 bool IsValidMediaSource(const MediaSource& source);
 
+// Extracts the presentation URL from |source|.
+// |source| must be based on a presentation URL, e.g. created from
+// ForPresentationUrl, and must be valid.
+std::string GetPresentationUrl(const MediaSource& source);
+
 }  // namespace media_router
 
 #endif  // CHROME_BROWSER_MEDIA_ROUTER_MEDIA_SOURCE_HELPER_H_