MacViews: draw combobox arrow backgrounds

ui/views/style/platform_style_mac.mm

Index: ui/views/style/platform_style_mac.mm
diff --git a/ui/views/style/platform_style_mac.mm b/ui/views/style/platform_style_mac.mm
index c9f4f8c6cde2702b89992028d613910b61e83411..1fb13f32a554ed486c3021b84e7bd534ac77104f 100644
--- a/ui/views/style/platform_style_mac.mm
+++ b/ui/views/style/platform_style_mac.mm
@@ -8,6 +8,7 @@
 #include "ui/views/controls/button/label_button_border.h"
 #include "ui/views/controls/focusable_rounded_border_mac.h"
 #import "ui/views/controls/scrollbar/cocoa_scroll_bar.h"
+#include "ui/views/style/mac/combobox_background_mac.h"
 #include "ui/views/style/mac/dialog_button_border_mac.h"
 
 namespace views {
@@ -18,6 +19,11 @@ scoped_ptr<FocusableBorder> PlatformStyle::CreateComboboxBorder() {
 }
 
 // static
+scoped_ptr<Background> PlatformStyle::CreateComboboxBackground() {
+  return make_scoped_ptr(new ComboboxBackgroundMac);
+}
+
+// static
 scoped_ptr<LabelButtonBorder> PlatformStyle::CreateLabelButtonBorder(
     Button::ButtonStyle style) {
   if (style == Button::STYLE_BUTTON)