Views: factor out Combobox background and const-ify style

ui/views/style/mac/combobox_background_mac.h

Index: ui/views/style/mac/combobox_background_mac.h
diff --git a/ui/views/style/mac/combobox_background_mac.h b/ui/views/style/mac/combobox_background_mac.h
index 5be50858d75f87e6c98a08ea52e3be7e80265fdc..353deab74d7236e531990ceaaf64fa3e4ddad39b 100644
--- a/ui/views/style/mac/combobox_background_mac.h
+++ b/ui/views/style/mac/combobox_background_mac.h
@@ -23,12 +23,17 @@ namespace views {
 // 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);
 };