[Media Router] Only keep track of toolbar MediaRouterAction in MediaRouterDialogControllerImpl.

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

Index: chrome/browser/ui/webui/media_router/media_router_dialog_controller_impl.h
diff --git a/chrome/browser/ui/webui/media_router/media_router_dialog_controller_impl.h b/chrome/browser/ui/webui/media_router/media_router_dialog_controller_impl.h
index f7545c3da07bcd3f0b1bfd88e93afd1f5fc67944..2ce9a2859ab91fbec18b282445017752831a1c9a 100644
--- a/chrome/browser/ui/webui/media_router/media_router_dialog_controller_impl.h
+++ b/chrome/browser/ui/webui/media_router/media_router_dialog_controller_impl.h
@@ -64,6 +64,13 @@ class MediaRouterDialogControllerImpl :
   // created.
   bool media_router_dialog_pending_;
 
+  // |action_| refers to the MediaRouterAction on the toolbar, rather than
+  // overflow menu. A MediaRouterAction is always created for the toolbar
+  // first. Any subsequent creations for the overflow menu will not be set as
+  // |action_|.
+  // The lifetime of |action_| is dependent on the creation and destruction of
+  // a browser window. The overflow menu's MediaRouterAction is only created
+  // when the overflow menu is opened and destroyed when the menu is closed.
   base::WeakPtr<MediaRouterAction> action_;
 
   DISALLOW_COPY_AND_ASSIGN(MediaRouterDialogControllerImpl);