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..d09e9da38144e7285bf317dfbdfb19f823f314f6 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 |shoulder_width| argument is the width of the "arrows" section from the
+  // diagram above, including any necessary padding around the actual arrow.
+  explicit ComboboxBackgroundMac(int shoulder_width);
   ~ComboboxBackgroundMac() override;
 
   // Background:
   void Paint(gfx::Canvas* canvas, View* view) const override;
+
  private:
+  int shoulder_width_;
+
   DISALLOW_COPY_AND_ASSIGN(ComboboxBackgroundMac);
 };