mash: Preliminary support for widget hit test masks

components/mus/ws/server_window.h

Index: components/mus/ws/server_window.h
diff --git a/components/mus/ws/server_window.h b/components/mus/ws/server_window.h
index 4baf37b2b1a5cde0420afc84ba6192581f8583c9..00ac6b5528dbc0f403fe3e0bf6469aa22c9595dc 100644
--- a/components/mus/ws/server_window.h
+++ b/components/mus/ws/server_window.h
@@ -82,6 +82,10 @@ class ServerWindow {
   void SetClientArea(const gfx::Insets& insets,
                      const std::vector<gfx::Rect>& additional_client_areas);
 
+  const gfx::Rect* hit_test_mask() const { return hit_test_mask_.get(); }
+  void SetHitTestMask(const gfx::Rect& mask);
+  void ClearHitTestMask();
+
   int32_t cursor() const { return static_cast<int32_t>(cursor_id_); }
   int32_t non_client_cursor() const {
     return static_cast<int32_t>(non_client_cursor_id_);
@@ -230,6 +234,10 @@ class ServerWindow {
   // amount.
   gfx::Insets extended_hit_test_region_;
 
+  // Mouse events outside the hit test mask don't hit the window. An empty mask
+  // means all events miss the window. If null there is no mask.
+  std::unique_ptr<gfx::Rect> hit_test_mask_;
+
   base::ObserverList<ServerWindowObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(ServerWindow);