MacViews: Clip contents for non-rectangular windows.

ui/views/cocoa/bridged_native_widget.h

Index: ui/views/cocoa/bridged_native_widget.h
diff --git a/ui/views/cocoa/bridged_native_widget.h b/ui/views/cocoa/bridged_native_widget.h
index 8f6523042de9eead7500964bd6d0daee8883ae63..4f28af828d2bb13f2c5a6c81f60514d96e20ad5a 100644
--- a/ui/views/cocoa/bridged_native_widget.h
+++ b/ui/views/cocoa/bridged_native_widget.h
@@ -215,6 +215,12 @@ class VIEWS_EXPORT BridgedNativeWidget
   // update its draggable region.
   void SetDraggable(bool draggable);
 
+  // Calls updateWindowMask on |bridged_view_| and updates the path
+  // corresponding to the mask of |compositor_superview_|'s layer for
+  // non-rectangular windows. If a mask layer does not yet exist, it creates a
+  // new one.
+  void UpdateMaskPath();
+
   // Overridden from CocoaMouseCaptureDelegate:
   void PostCapturedEvent(NSEvent* event) override;
   void OnMouseCaptureLost() override;