Refactor backdrop that is currently used in the maximized mode.

ui/aura/client/aura_constants.h

Index: ui/aura/client/aura_constants.h
diff --git a/ui/aura/client/aura_constants.h b/ui/aura/client/aura_constants.h
index 04af9214cabb7c4ef725792c52930e8f9a8c1b97..361186b933c792704790e4897fb03fdf8cd9a878 100644
--- a/ui/aura/client/aura_constants.h
+++ b/ui/aura/client/aura_constants.h
@@ -68,6 +68,10 @@ AURA_EXPORT extern const WindowProperty<bool>* const kDrawAttentionKey;
 // A property key to store the focus client on the window.
 AURA_EXPORT extern const WindowProperty<FocusClient*>* const kFocusClientKey;
 
+// A bool property key to specify if the window should have a backdrop window
+// (typically black) that covers the desktop behind the window.
+AURA_EXPORT extern const WindowProperty<bool>* const kHasBackdrop;
+
 // A property key to store the host window of a window. This lets
 // WebContentsViews find the windows that should constrain NPAPI plugins.
 AURA_EXPORT extern const WindowProperty<Window*>* const kHostWindowKey;