Add app.window.alphaEnabled() and onAlphaEnabledChanged.

ui/views/widget/widget.h

Index: ui/views/widget/widget.h
diff --git a/ui/views/widget/widget.h b/ui/views/widget/widget.h
index 6f85324bdd9325ba00f38ad5cb8ed777fee30fc1..d4770b5fb3874f3b2c96d7687c4555c81e00938e 100644
--- a/ui/views/widget/widget.h
+++ b/ui/views/widget/widget.h
@@ -198,6 +198,9 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
     WidgetDelegate* delegate;
     bool child;
     // If TRANSLUCENT_WINDOW, the widget may be fully or partially transparent.
+    // Translucent windows may not always be supported. Use
+    // IsTranslucentWindowOpacitySupported to determine if translucent windows
+    // are supported.
     // If OPAQUE_WINDOW, we can perform optimizations based on the widget being
     // fully opaque.  Defaults to TRANSLUCENT_WINDOW if
     // ViewsDelegate::UseTransparentWindows().  Defaults to OPAQUE_WINDOW for
@@ -726,6 +729,9 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
   // window sizing information to the window server on some platforms.
   void OnRootViewLayout();
 
+  // Whether the widget supports translucency.
+  bool IsTranslucentWindowOpacitySupported() const;
+
   // Notification that our owner is closing.
   // NOTE: this is not invoked for aura as it's currently not needed there.
   // Under aura menus close by way of activation getting reset when the owner