MacViews: draw combobox arrow backgrounds

ui/views/style/platform_style_mac.mm

 // 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.
 
 #include "ui/views/style/platform_style.h"
 
 #include "ui/views/controls/button/label_button.h"
 #include "ui/views/controls/button/label_button_border.h"
 #include "ui/views/controls/focusable_rounded_border_mac.h"
 #import "ui/views/controls/scrollbar/cocoa_scroll_bar.h"
+#include "ui/views/style/mac/combobox_background_mac.h"
 #include "ui/views/style/mac/dialog_button_border_mac.h"
 
 namespace views {
 
 // static
 scoped_ptr<FocusableBorder> PlatformStyle::CreateComboboxBorder() {
   return make_scoped_ptr(new FocusableRoundedBorder);
 }
 
 // static
+scoped_ptr<Background> PlatformStyle::CreateComboboxBackground() {
+  return make_scoped_ptr(new ComboboxBackgroundMac);
+}
+
+// static
 scoped_ptr<LabelButtonBorder> PlatformStyle::CreateLabelButtonBorder(
     Button::ButtonStyle style) {
   if (style == Button::STYLE_BUTTON)
     return make_scoped_ptr(new DialogButtonBorderMac());
 
   return make_scoped_ptr(new LabelButtonAssetBorder(style));
 }
 
 // static
 scoped_ptr<ScrollBar> PlatformStyle::CreateScrollBar(bool is_horizontal) {
   return make_scoped_ptr(new CocoaScrollBar(is_horizontal));
 }
 
 }  // namespace views