Views: factor out Combobox background and const-ify style

ui/views/style/platform_style.h

 // 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.
 
 #ifndef UI_VIEWS_STYLE_PLATFORM_STYLE_H_
 #define UI_VIEWS_STYLE_PLATFORM_STYLE_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 31 matching lines @@
   // The |is_enabled| argument is true if the control the arrow is for is
   // enabled, and false if the control is disabled. The |style| argument is the
   // style of the combobox the arrow is being drawn for.
   static gfx::ImageSkia CreateComboboxArrow(bool is_enabled,
                                             Combobox::Style style);
 
   // Creates the appropriate border for a focusable Combobox.
   static std::unique_ptr<FocusableBorder> CreateComboboxBorder();
 
   // Creates the appropriate background for a Combobox.
-  static std::unique_ptr<Background> CreateComboboxBackground();
+  static std::unique_ptr<Background> CreateComboboxBackground(
+      int shoulder_width);
 
   // Creates the default label button border for the given |style|. Used when a
   // custom default border is not provided for a particular LabelButton class.
   static std::unique_ptr<LabelButtonBorder> CreateLabelButtonBorder(
       Button::ButtonStyle style);
 
   // Creates the default scrollbar for the given orientation.
   static std::unique_ptr<ScrollBar> CreateScrollBar(bool is_horizontal);
 
   // Returns the current text color for the current button state.
@@ skipping 10 matching lines @@
   static std::unique_ptr<Border> CreateThemedLabelButtonBorder(
       LabelButton* button);
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PlatformStyle);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_STYLE_PLATFORM_STYLE_H_