Views: factor out Combobox background and const-ify style

ui/views/style/platform_style.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.
 
 #include "ui/views/style/platform_style.h"
 
 #include "base/memory/ptr_util.h"
 #include "build/build_config.h"
 #include "ui/base/material_design/material_design_controller.h"
 #include "ui/base/resource/resource_bundle.h"
@@ skipping 34 matching lines @@
   ui::ResourceBundle& rb = ui::ResourceBundle::GetSharedInstance();
   return *rb.GetImageSkiaNamed(IDR_MENU_DROPARROW);
 }
 
 // static
 std::unique_ptr<FocusableBorder> PlatformStyle::CreateComboboxBorder() {
   return base::WrapUnique(new FocusableBorder());
 }
 
 // static
-std::unique_ptr<Background> PlatformStyle::CreateComboboxBackground() {
+std::unique_ptr<Background> PlatformStyle::CreateComboboxBackground(
+    int shoulder_width) {
   return nullptr;
 }
 
 // static
 std::unique_ptr<LabelButtonBorder> PlatformStyle::CreateLabelButtonBorder(
     Button::ButtonStyle style) {
   if (!ui::MaterialDesignController::IsModeMaterial() ||
       style != Button::STYLE_TEXTBUTTON) {
     return base::WrapUnique(new LabelButtonAssetBorder(style));
   }
@@ skipping 39 matching lines @@
 
 #if !defined(DESKTOP_LINUX)
 // static
 std::unique_ptr<Border> PlatformStyle::CreateThemedLabelButtonBorder(
     LabelButton* button) {
   return button->CreateDefaultBorder();
 }
 #endif
 
 }  // namespace views