cros: CustomFrameViewAsh avatar icon from a window prop

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 361186b933c792704790e4897fb03fdf8cd9a878..cfe7c20f5ef9f0d3340cb547a199fe70acae1629 100644
--- a/ui/aura/client/aura_constants.h
+++ b/ui/aura/client/aura_constants.h
@@ -55,6 +55,10 @@ AURA_EXPORT extern const WindowProperty<gfx::ImageSkia*>* const kAppIconKey;
 // pointer metrics. See AppType in ash/shared/app_types.h for more details.
 AURA_EXPORT extern const WindowProperty<int>* const kAppType;
 
+// A property key to store the avatar icon that will be displayed on the window
+// frame to indicate the owner of the window when needed.
+AURA_EXPORT extern const WindowProperty<gfx::ImageSkia*>* const kAvatarIconKey;
+
 // A property key to store if a window is a constrained window or not.
 AURA_EXPORT extern const WindowProperty<bool>* const kConstrainedWindowKey;