WIP: Eliminate event.target() usage from TrayEventFilter

ui/views/mus/native_widget_mus.cc

Index: ui/views/mus/native_widget_mus.cc
diff --git a/ui/views/mus/native_widget_mus.cc b/ui/views/mus/native_widget_mus.cc
index 7bcc4ac7a17206250df694f2adab5f9ba3fdd336..7c80b6ab60ba47edcadef98c010fae9d6b1f66ef 100644
--- a/ui/views/mus/native_widget_mus.cc
+++ b/ui/views/mus/native_widget_mus.cc
@@ -377,6 +377,16 @@ void NativeWidgetMus::NotifyFrameChanged(
   }
 }
 
+// static
+Widget* NativeWidgetMus::GetWidgetForWindow(mus::Window* window) {
+  if (!window)
+    return nullptr;
+  NativeWidgetMus* native_widget = window->GetLocalProperty(kNativeWidgetMusKey);
+  if (!native_widget)
+    return nullptr;
+  return native_widget->GetWidget();
+}
+
 aura::Window* NativeWidgetMus::GetRootWindow() {
   return window_tree_host_->window();
 }