Linux Aura: Fix overlay shadows on notifications

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 <map>
 #include <memory>
 #include <set>
@@ skipping 147 matching lines @@
       TYPE_MENU,        // An undecorated Window, with transient properties
                         // specialized to menus.
       TYPE_TOOLTIP,
       TYPE_BUBBLE,
       TYPE_DRAG,        // An undecorated Window, used during a drag-and-drop to
                         // show the drag image.
     };
 
     enum WindowOpacity {
       // Infer fully opaque or not. For WinAura, top-level windows that are not
-      // of TYPE_WINDOW are translucent so that they can be made to fade in. In
-      // all other cases, windows are fully opaque.
+      // of TYPE_WINDOW are translucent so that they can be made to fade in.
+      // For LinuxAura, only windows that are TYPE_DRAG are translucent.  In all
+      // other cases, windows are fully opaque.
       INFER_OPACITY,
       // Fully opaque.
       OPAQUE_WINDOW,
-      // Possibly translucent/transparent.
+      // Possibly translucent/transparent.  Widgets that fade in or out using
+      // SetOpacity() but do not make use of an alpha channel should use
+      // INFER_OPACITY.
       TRANSLUCENT_WINDOW,
     };
 
     enum Activatable {
       // Infer whether the window should be activatable from the window type.
       ACTIVATABLE_DEFAULT,
 
       ACTIVATABLE_YES,
       ACTIVATABLE_NO
     };
@@ skipping 783 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_