Views: factor out Combobox background and const-ify style

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 "base/memory/ptr_util.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/gfx/paint_vector_icon.h"
 #include "ui/gfx/vector_icons.h"
@@ skipping 26 matching lines @@
                                kComboboxArrowWidth,
                                is_enabled ? SK_ColorWHITE : SK_ColorBLACK);
 }
 
 // static
 std::unique_ptr<FocusableBorder> PlatformStyle::CreateComboboxBorder() {
   return base::WrapUnique(new FocusableRoundedBorder);
 }
 
 // static
-std::unique_ptr<Background> PlatformStyle::CreateComboboxBackground() {
-  return base::WrapUnique(new ComboboxBackgroundMac);
+std::unique_ptr<Background> PlatformStyle::CreateComboboxBackground(
+    int shoulder_width) {
+  return base::WrapUnique(new ComboboxBackgroundMac(shoulder_width));
 }
 
 // static
 std::unique_ptr<LabelButtonBorder> PlatformStyle::CreateLabelButtonBorder(
     Button::ButtonStyle style) {
   if (style == Button::STYLE_BUTTON)
     return base::WrapUnique(new DialogButtonBorderMac());
 
   return base::WrapUnique(new LabelButtonAssetBorder(style));
 }
@@ skipping 21 matching lines @@
 void PlatformStyle::ApplyLabelButtonTextStyle(
     views::Label* label,
     ButtonColorByState* color_by_state) {
   const ui::NativeTheme* theme = label->GetNativeTheme();
   ButtonColorByState& colors = *color_by_state;
   colors[Button::STATE_PRESSED] =
       theme->GetSystemColor(ui::NativeTheme::kColorId_ButtonHighlightColor);
 }
 
 }  // namespace views