use a black brush instead of a null brush for window background

ui/gfx/win/window_impl.cc

 // Copyright (c) 2012 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/gfx/win/window_impl.h"
 
 #include <list>
 
 #include "base/debug/alias.h"
 #include "base/memory/singleton.h"
@@ skipping 90 matching lines @@
       base::IntToString16(registered_count_++);
 
   WNDCLASSEX window_class;
   base::win::InitializeWindowClass(
       name.c_str(),
       &base::win::WrappedWindowProc<WindowImpl::WndProc>,
       class_info.style,
       0,
       0,
       NULL,
-      NULL,
+      reinterpret_cast<HBRUSH>(GetStockObject(BLACK_BRUSH)),
       NULL,
       class_info.icon,
       class_info.icon,
       &window_class);
   HMODULE instance = window_class.hInstance;
   ATOM atom = RegisterClassEx(&window_class);
   CHECK(atom) << GetLastError();
 
   registered_classes_.push_back(RegisteredClass(class_info, atom));
 
@@ skipping 141 matching lines @@
   return window->OnWndProc(message, w_param, l_param);
 }
 
 ATOM WindowImpl::GetWindowClassAtom() {
   HICON icon = GetDefaultWindowIcon();
   ClassInfo class_info(initial_class_style(), icon);
   return ClassRegistrar::GetInstance()->RetrieveClassAtom(class_info);
 }
 
 }  // namespace gfx