Centralizes more shared code between ash and mash

ash/mus/bridge/wm_shelf_mus.cc

Index: ash/mus/bridge/wm_shelf_mus.cc
diff --git a/ash/mus/bridge/wm_shelf_mus.cc b/ash/mus/bridge/wm_shelf_mus.cc
index c8ee95c08ae060a4f10dd66140438d251d8a41e0..1ef54fa9ddc59d2efb17f6cebdf16b547ee0b6f2 100644
--- a/ash/mus/bridge/wm_shelf_mus.cc
+++ b/ash/mus/bridge/wm_shelf_mus.cc
@@ -21,11 +21,7 @@ WmShelfMus::WmShelfMus(WmWindow* root_window) {
   shelf_widget()->PostCreateShelf();
 }
 
-WmShelfMus::~WmShelfMus() {
-  ShutdownShelfWidget();
-  DestroyShelfWidget();
-  ShutdownShelf();
-}
+WmShelfMus::~WmShelfMus() {}
 
 void WmShelfMus::WillDeleteShelfLayoutManager() {
   ShutdownShelfWidget();