Add app.window.alphaEnabled() and onAlphaEnabledChanged.

apps/ui/native_app_window.h

Index: apps/ui/native_app_window.h
diff --git a/apps/ui/native_app_window.h b/apps/ui/native_app_window.h
index cde4c1dfad8102de7ce4b484e60c1fcc30e9e5b5..fd0cc2633718e7523be5e5edf81fc7dfcffe44f6 100644
--- a/apps/ui/native_app_window.h
+++ b/apps/ui/native_app_window.h
@@ -87,6 +87,10 @@ class NativeAppWindow : public ui::BaseWindow,
   virtual void SetContentSizeConstraints(const gfx::Size& min_size,
                                          const gfx::Size& max_size) = 0;
 
+  // Returns false if the underlying native window ignores alpha transparency
+  // when compositing.
+  virtual bool CanHaveAlphaEnabled() const = 0;
+
   virtual ~NativeAppWindow() {}
 };