Remove (Native)Widget::StackBelow().

ui/views/widget/native_widget_mac.mm

 // Copyright 2014 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 "ui/views/widget/native_widget_mac.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include <utility>
 
@@ skipping 314 matching lines @@
 }
 
 void NativeWidgetMac::StackAbove(gfx::NativeView native_view) {
   NOTIMPLEMENTED();
 }
 
 void NativeWidgetMac::StackAtTop() {
   NOTIMPLEMENTED();
 }
 
-void NativeWidgetMac::StackBelow(gfx::NativeView native_view) {
-  NOTIMPLEMENTED();
-}
-
 void NativeWidgetMac::SetShape(std::unique_ptr<SkRegion> shape) {
   NOTIMPLEMENTED();
 }
 
 void NativeWidgetMac::Close() {
   if (!bridge_)
     return;
 
   NSWindow* window = GetNativeWindow();
   if (IsWindowModalSheet()) {
@@ skipping 413 matching lines @@
   [[ViewsNSWindowCloseAnimator alloc] initWithWindow:window];
 }
 
 - (void)animationDidEnd:(NSAnimation*)animation {
   [window_ close];
   [animation_ setDelegate:nil];
   [self release];
 }
 
 @end