MacViews: Add native drop shadow to dialogs on OSX < 10.10.

ui/views/cocoa/bridged_content_view.h

Index: ui/views/cocoa/bridged_content_view.h
diff --git a/ui/views/cocoa/bridged_content_view.h b/ui/views/cocoa/bridged_content_view.h
index 9f6fbb505661a5a84985412dbe16f3958c7957e7..cd682cb946d7d42e49585b2ce6a19c3aa22c70d6 100644
--- a/ui/views/cocoa/bridged_content_view.h
+++ b/ui/views/cocoa/bridged_content_view.h
@@ -47,6 +47,11 @@ class View;
 
   // Whether dragging on the view moves the window.
   BOOL mouseDownCanMoveWindow_;
+
+  // The current window mask of the window to which this view is bound. For
+  // windows with rectangular shapes and on Mac OS versions later than 10.9,
+  // this is not used and is nil.
+  base::scoped_nsobject<NSBezierPath> windowMask_;
 }
 
 @property(readonly, nonatomic) views::View* hostedView;