MacViews: draw combobox arrow backgrounds

ui/views/style/platform_style.cc

Index: ui/views/style/platform_style.cc
diff --git a/ui/views/style/platform_style.cc b/ui/views/style/platform_style.cc
index e5e43e00a99a3e04d141339cf0c63c35813edbae..596112b9648b486d976e2064cbd40db74c368430 100644
--- a/ui/views/style/platform_style.cc
+++ b/ui/views/style/platform_style.cc
@@ -6,6 +6,7 @@
 
 #include "build/build_config.h"
 #include "ui/base/material_design/material_design_controller.h"
+#include "ui/views/background.h"
 #include "ui/views/controls/button/label_button.h"
 #include "ui/views/controls/button/label_button_border.h"
 #include "ui/views/controls/focusable_border.h"
@@ -14,12 +15,18 @@
 namespace views {
 
 #if !defined(OS_MACOSX)
+
 // static
 scoped_ptr<FocusableBorder> PlatformStyle::CreateComboboxBorder() {
   return make_scoped_ptr(new FocusableBorder());
 }
 
 // static
+scoped_ptr<Background> PlatformStyle::CreateComboboxBackground() {
+  return nullptr;
+}
+
+// static
 scoped_ptr<LabelButtonBorder> PlatformStyle::CreateLabelButtonBorder(
     Button::ButtonStyle style) {
   if (!ui::MaterialDesignController::IsModeMaterial() ||