Add app.window.alphaEnabled() and onAlphaEnabledChanged.

ui/views/widget/widget.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_VIEWS_WIDGET_WIDGET_H_
 #define UI_VIEWS_WIDGET_WIDGET_H_
 
 #include <set>
 #include <stack>
 #include <vector>
@@ skipping 180 matching lines @@
 
     InitParams();
     explicit InitParams(Type type);
     ~InitParams();
 
     Type type;
     // If NULL, a default implementation will be constructed.
     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
     // non-window widgets.
     WindowOpacity opacity;
     bool accept_events;
     Activatable activatable;
     bool keep_on_top;
     bool visible_on_all_workspaces;
     Ownership ownership;
@@ skipping 508 matching lines @@
   gfx::Rect GetWorkAreaBoundsInScreen() const;
 
   // Creates and dispatches synthesized mouse move event using the current
   // mouse location to refresh hovering status in the widget.
   void SynthesizeMouseMoveEvent();
 
   // Called by our RootView after it has performed a Layout. Used to forward
   // 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
   // closes.
   virtual void OnOwnerClosing();
 
   // Overridden from NativeWidgetDelegate:
   virtual bool IsModal() const OVERRIDE;
   virtual bool IsDialogBox() const OVERRIDE;
   virtual bool CanActivate() const OVERRIDE;
@@ skipping 195 matching lines @@
   bool movement_disabled_;
 
   ScopedObserver<ui::NativeTheme, ui::NativeThemeObserver> observer_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(Widget);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_WIDGET_H_