Views: factor out Combobox background and const-ify style

ui/views/style/mac/combobox_background_mac.h

 // Copyright 2016 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_MAC_COMBOBOX_BACKGROUND_MAC_H_
 #define UI_VIEWS_STYLE_MAC_COMBOBOX_BACKGROUND_MAC_H_
 
 #include "base/macros.h"
 #include "ui/views/background.h"
 
 namespace gfx {
 class Canvas;
 }
 
 namespace views {
 
 // This class implements the background of comboboxes. Comboboxes look like
 // this:
 //   ---------------------------
 //   | Option Text... | Arrows |
 //   ---------------------------
 // This class colors the background of the arrows section in accordance with our
 // Mac look and feel.
 class ComboboxBackgroundMac : public Background {
  public:
-  ComboboxBackgroundMac();
+  // The |container_width| argument is the width of the "arrows" section from
+  // the diagram above, including any necessary padding around the actual arrow.
+  explicit ComboboxBackgroundMac(int container_width);
   ~ComboboxBackgroundMac() override;
 
   // Background:
   void Paint(gfx::Canvas* canvas, View* view) const override;
+
  private:
+  int container_width_;
+
   DISALLOW_COPY_AND_ASSIGN(ComboboxBackgroundMac);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_STYLE_MAC_COMBOBOX_BACKGROUND_MAC_H_