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

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 7971b0f559f543a891eb28ddd35d5b6731eeb8cb..f56328c73c76dbb7467176f370b1499ed7d495b4 100644
--- a/ash/mus/bridge/wm_shelf_mus.cc
+++ b/ash/mus/bridge/wm_shelf_mus.cc
@@ -64,8 +64,13 @@ ShelfBackgroundType WmShelfMus::GetBackgroundType() const {
   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;
 }