mash shelf: Don't create shelf DimmerView in ShelfWidget on mus

ash/mus/bridge/wm_shelf_mus.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/mus/bridge/wm_shelf_mus.h"
 
 #include "ash/mus/bridge/wm_window_mus.h"
 #include "ash/mus/shelf_layout_manager.h"
 #include "services/ui/public/cpp/window.h"
 
@@ skipping 46 matching lines @@
 
 void WmShelfMus::UpdateAutoHideState() {
   NOTIMPLEMENTED();
 }
 
 ShelfBackgroundType WmShelfMus::GetBackgroundType() const {
   NOTIMPLEMENTED();
   return SHELF_BACKGROUND_DEFAULT;
 }
 
+WmDimmerView* WmShelfMus::CreateDimmerView(bool disable_animations_for_test) {
+  // mus does not dim shelf items.
+  return nullptr;
+}
+
 bool WmShelfMus::IsDimmed() const {
-  NOTIMPLEMENTED();
+  // mus does not dim shelf items.
   return false;
 }
 
 bool WmShelfMus::IsShowingOverflowBubble() const {
   NOTIMPLEMENTED();
   return false;
 }
 
 void WmShelfMus::SchedulePaint() {
   NOTIMPLEMENTED();
@@ skipping 54 matching lines @@
   return nullptr;
 }
 
 ShelfView* WmShelfMus::GetShelfViewForTesting() {
   NOTIMPLEMENTED();
   return nullptr;
 }
 
 }  // namespace mus
 }  // namespace ash