[Media Router] Force DEFAULT cast mode when starting presentations from content.

chrome/browser/ui/webui/media_router/media_cast_mode.h

Index: chrome/browser/ui/webui/media_router/media_cast_mode.h
diff --git a/chrome/browser/ui/webui/media_router/media_cast_mode.h b/chrome/browser/ui/webui/media_router/media_cast_mode.h
index 6167c0f638e012efb459369c23ccd49143edd101..9a3a7baa7fd517f29b2fd3d1406874c4deddb25a 100644
--- a/chrome/browser/ui/webui/media_router/media_cast_mode.h
+++ b/chrome/browser/ui/webui/media_router/media_cast_mode.h
@@ -14,8 +14,11 @@ namespace media_router {
 // be presented to a media sink.  These must be declared in the priority order
 // returned by GetPreferredCastMode.
 enum MediaCastMode {
-  // The default presentation for the WebContents.  Only available when the
-  // document has provided a default presentation URL.
+  // A presentation URL provided by the WebContents via the Presentation API.
+  // This can be set by the default presentation URL (for top level browsing
+  // contexts) or presentation URLs passed with a PresentationRequest (for top
+  // level and nested browsing contexts).
+  // TODO(mfoltz): More accurately named PRESENTATION
   DEFAULT = 0x1,
   // Capture the rendered WebContents and stream it to a media sink.  Always
   // available.