WIP: Eliminate event.target() usage from TrayEventFilter

ui/views/mus/native_widget_mus.h

Index: ui/views/mus/native_widget_mus.h
diff --git a/ui/views/mus/native_widget_mus.h b/ui/views/mus/native_widget_mus.h
index c0cc1957482353c8087bf9e40f431572616befac..10f155b82e5c3cbbd423eeb38dc9b949f2bf0218 100644
--- a/ui/views/mus/native_widget_mus.h
+++ b/ui/views/mus/native_widget_mus.h
@@ -73,6 +73,9 @@ class VIEWS_MUS_EXPORT NativeWidgetMus : public internal::NativeWidgetPrivate,
   // Notifies all widgets the frame constants changed in some way.
   static void NotifyFrameChanged(mus::WindowTreeConnection* connection);
 
+  // Returns the widget for a mus::Window, or null if there is none.
+  static Widget* GetWidgetForWindow(mus::Window* window);
+
   mus::Window* window() { return window_; }
   WindowTreeHostMus* window_tree_host() { return window_tree_host_.get(); }