Refactor ViewsDelegate singleton

ui/views/test/test_views_delegate_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/test/test_views_delegate.h"
 
 #include "ui/views/widget/native_widget_mac.h"
 
 namespace views {
 
 TestViewsDelegate::TestViewsDelegate()
     : context_factory_(nullptr),
       use_desktop_native_widgets_(false),
       use_transparent_windows_(false) {
-  DCHECK(!ViewsDelegate::views_delegate);
-  ViewsDelegate::views_delegate = this;
 }
 
 TestViewsDelegate::~TestViewsDelegate() {
-  if (ViewsDelegate::views_delegate == this)
-    ViewsDelegate::views_delegate = NULL;
 }
 
 void TestViewsDelegate::OnBeforeWidgetInit(
     Widget::InitParams* params,
     internal::NativeWidgetDelegate* delegate) {
   if (params->opacity == Widget::InitParams::INFER_OPACITY) {
     params->opacity = use_transparent_windows_ ?
         Widget::InitParams::TRANSLUCENT_WINDOW :
         Widget::InitParams::OPAQUE_WINDOW;
   }
   // TODO(tapted): This should return a *Desktop*NativeWidgetMac.
   if (!params->native_widget && use_desktop_native_widgets_)
     params->native_widget = new NativeWidgetMac(delegate);
 }
 
 ui::ContextFactory* TestViewsDelegate::GetContextFactory() {
   return context_factory_;
 }
 
 }  // namespace views