mash: Preliminary support for widget hit test masks

components/mus/public/interfaces/window_tree.mojom

Index: components/mus/public/interfaces/window_tree.mojom
diff --git a/components/mus/public/interfaces/window_tree.mojom b/components/mus/public/interfaces/window_tree.mojom
index e5a15f16c23aee0782e43a591ccf288a242eacd7..045a943623ebdfc928726bac68b78a8445c61aa6 100644
--- a/components/mus/public/interfaces/window_tree.mojom
+++ b/components/mus/public/interfaces/window_tree.mojom
@@ -129,6 +129,11 @@ interface WindowTree {
                 mojo.Insets insets,
                 array<mojo.Rect>? additional_client_areas);
 
+  // Mouse events outside a hit test mask do not hit the window. The |mask| is
+  // in window local coordinates. Pass null to clear the mask.
+  // TODO(jamescook): Convert |mask| to a path. http://crbug.com/613210
+  SetHitTestMask(uint32 window_id, mojo.Rect? mask);
+
   // Sets the visibility of the specified window to |visible|. Connections are
   // allowed to change the visibility of any window they have created, as well
   // as any of their roots.