Remove (Native)Widget::StackBelow().

ui/views/mus/native_widget_mus.cc

 // Copyright 2015 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.
 
 // This has to be before any other includes, else default is picked up.
 // See base/logging for details on this.
 #define NOTIMPLEMENTED_POLICY 5
 
 #include "ui/views/mus/native_widget_mus.h"
 
@@ skipping 992 matching lines @@
 }
 
 void NativeWidgetMus::StackAbove(gfx::NativeView native_view) {
   NOTIMPLEMENTED();
 }
 
 void NativeWidgetMus::StackAtTop() {
   NOTIMPLEMENTED();
 }
 
-void NativeWidgetMus::StackBelow(gfx::NativeView native_view) {
-  NOTIMPLEMENTED();
-}
-
 void NativeWidgetMus::SetShape(std::unique_ptr<SkRegion> shape) {
   NOTIMPLEMENTED();
 }
 
 void NativeWidgetMus::Close() {
   Hide();
   if (!close_widget_factory_.HasWeakPtrs()) {
     base::ThreadTaskRunnerHandle::Get()->PostTask(
         FROM_HERE, base::Bind(&NativeWidgetMus::CloseNow,
                               close_widget_factory_.GetWeakPtr()));
@@ skipping 509 matching lines @@
 
   gfx::Path mask_path;
   native_widget_delegate_->GetHitTestMask(&mask_path);
   // TODO(jamescook): Use the full path for the mask.
   gfx::Rect mask_rect =
       gfx::ToEnclosingRect(gfx::SkRectToRectF(mask_path.getBounds()));
   window_->SetHitTestMask(mask_rect);
 }
 
 }  // namespace views