mash: Migrate ShelfButton* to wm common types

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 124 matching lines @@
 
 void WmShelfMus::SetKeyboardBoundsForTesting(const gfx::Rect& bounds) {
   NOTIMPLEMENTED();
 }
 
 ShelfLockingManager* WmShelfMus::GetShelfLockingManagerForTesting() {
   NOTIMPLEMENTED();
   return nullptr;
 }
 
+ShelfView* WmShelfMus::GetShelfViewForTesting() {
+  NOTIMPLEMENTED();
+  return nullptr;
+}
+
 }  // namespace mus
 }  // namespace ash