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

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

Index: chrome/browser/ui/webui/media_router/media_router_ui.h
diff --git a/chrome/browser/ui/webui/media_router/media_router_ui.h b/chrome/browser/ui/webui/media_router/media_router_ui.h
index 04a6a7c7049a1d8dd4e215244b798a4b491ea44c..ea9474e6c49dd090763010d131cedf1fa5e0f555 100644
--- a/chrome/browser/ui/webui/media_router/media_router_ui.h
+++ b/chrome/browser/ui/webui/media_router/media_router_ui.h
@@ -152,6 +152,9 @@ class MediaRouterUI : public ConstrainedWebDialogUI,
     return routes_and_cast_modes_;
   }
   const content::WebContents* initiator() const { return initiator_; }
+  const base::Optional<MediaCastMode>& forced_cast_mode() const {
+    return forced_cast_mode_;
+  }
 
   virtual const std::string& GetRouteProviderExtensionId() const;
 
@@ -393,6 +396,9 @@ class MediaRouterUI : public ConstrainedWebDialogUI,
   // updates.
   std::unique_ptr<UIMediaRouteControllerObserver> route_controller_observer_;
 
+  // If set, a cast mode that is required to be shown first.
+  base::Optional<MediaCastMode> forced_cast_mode_;
+
   // NOTE: Weak pointers must be invalidated before all other member variables.
   // Therefore |weak_factory_| must be placed at the end.
   base::WeakPtrFactory<MediaRouterUI> weak_factory_;