mash: Preliminary support for widget hit test masks

components/mus/public/cpp/window.h

Index: components/mus/public/cpp/window.h
diff --git a/components/mus/public/cpp/window.h b/components/mus/public/cpp/window.h
index 9ce601f291c97c90d3924535034e37c1a6e75d78..b69180cc3996f87a57a00356c845d2adf497ed9f 100644
--- a/components/mus/public/cpp/window.h
+++ b/components/mus/public/cpp/window.h
@@ -86,6 +86,10 @@ class Window {
   void SetClientArea(const gfx::Insets& new_client_area,
                      const std::vector<gfx::Rect>& additional_client_areas);
 
+  // Mouse events outside the hit test mask do not hit the window.
+  const gfx::Rect& hit_test_mask() const { return hit_test_mask_; }
+  void SetHitTestMask(const gfx::Rect& mask_rect);
+
   // Visibility (also see IsDrawn()). When created windows are hidden.
   bool visible() const { return visible_; }
   void SetVisible(bool value);
@@ -321,6 +325,7 @@ class Window {
   gfx::Rect bounds_;
   gfx::Insets client_area_;
   std::vector<gfx::Rect> additional_client_areas_;
+  gfx::Rect hit_test_mask_;
 
   mojom::ViewportMetricsPtr viewport_metrics_;