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..6f0b2b574866086e82b4fddce72b22bb34efdf49 100644
--- a/ui/views/cocoa/bridged_native_widget.h
+++ b/ui/views/cocoa/bridged_native_widget.h
@@ -215,6 +215,11 @@ class VIEWS_EXPORT BridgedNativeWidget
   // update its draggable region.
   void SetDraggable(bool draggable);
 
+  // For non-rectangular windows, updates the path corresponding to the mask of
+  // |compositor_superview_|'s layer. 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;