Use Ash's ShelfWindowWatcher for app panel windows.

ash/mus/bridge/wm_window_mus.cc

Index: ash/mus/bridge/wm_window_mus.cc
diff --git a/ash/mus/bridge/wm_window_mus.cc b/ash/mus/bridge/wm_window_mus.cc
index b35cad1fd4bd3de46a038bc37038175c43b06441..f9830cf068b352f7e1c8b6e6ff6d05b3916734a9 100644
--- a/ash/mus/bridge/wm_window_mus.cc
+++ b/ash/mus/bridge/wm_window_mus.cc
@@ -491,6 +491,16 @@ void WmWindowMus::SetIntProperty(WmWindowProperty key, int value) {
   NOTREACHED();
 }
 
+gfx::ImageSkia WmWindowMus::GetWindowIcon() {
+  NOTIMPLEMENTED();
+  return gfx::ImageSkia();
+}
+
+gfx::ImageSkia WmWindowMus::GetAppIcon() {
+  NOTIMPLEMENTED();
+  return gfx::ImageSkia();
+}
+
 const wm::WindowState* WmWindowMus::GetWindowState() const {
   return window_state_.get();
 }