Add app.window.alphaEnabled() and onAlphaEnabledChanged.

ui/views/widget/desktop_aura/desktop_window_tree_host.h

Index: ui/views/widget/desktop_aura/desktop_window_tree_host.h
diff --git a/ui/views/widget/desktop_aura/desktop_window_tree_host.h b/ui/views/widget/desktop_aura/desktop_window_tree_host.h
index 26925b672ff14657c27e70c651a01e2a888ff7c4..a9fb883a9b03126e86878108aae57ea8e6ce576d 100644
--- a/ui/views/widget/desktop_aura/desktop_window_tree_host.h
+++ b/ui/views/widget/desktop_aura/desktop_window_tree_host.h
@@ -153,6 +153,9 @@ class VIEWS_EXPORT DesktopWindowTreeHost {
   // Returns true if the Widget was closed but is still showing because of
   // animations.
   virtual bool IsAnimatingClosed() const = 0;
+
+  // Returns true if the Widget can have a transparent background.
+  virtual bool CanHaveAlphaEnabled() const = 0;
 };
 
 }  // namespace views