Use Ash's ShelfWindowWatcher for app panel windows.

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 3a054c2b4d3cae9971d5375e2c796ee396d1569a..55415a3d918acf0107d7cf3ad9d9b7f9cc8a7b0c 100644
--- a/ui/aura/client/aura_constants.h
+++ b/ui/aura/client/aura_constants.h
@@ -83,9 +83,12 @@ AURA_EXPORT extern const aura::WindowProperty<int>* const kTopViewInset;
 // The color of the window header.
 AURA_EXPORT extern const aura::WindowProperty<SkColor>* const kTopViewColor;
 
-// A property key to store window icon.
+// A property key to store the window icon, typically 16x16 for title bars.
 AURA_EXPORT extern const WindowProperty<gfx::ImageSkia*>* const kWindowIconKey;
 
+// A property key to store the app icon, typically larger for shelf icons, etc.
+AURA_EXPORT extern const WindowProperty<gfx::ImageSkia*>* const kAppIconKey;
+
 // Alphabetical sort.
 
 }  // namespace client