mash: Preliminary support for widget hit test masks

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 7532719db35e7a0532bd40dc2181af36c412d071..f8ce3e2f0bcc3fca6178c47d971a674f6d55d5b9 100644
--- a/ui/views/mus/native_widget_mus.h
+++ b/ui/views/mus/native_widget_mus.h
@@ -238,6 +238,11 @@ private:
   void OnMusWindowVisibilityChanging(mus::Window* window);
   void OnMusWindowVisibilityChanged(mus::Window* window);
 
+  // Propagates the widget hit test mask, if any, to the mus::Window.
+  // TODO(jamescook): Wire this through views::Widget so widgets can push
+  // updates if needed.
+  void UpdateHitTestMask();
+
   mus::Window* window_;
   mus::mojom::Cursor last_cursor_;