MacViews: draw combobox arrow backgrounds

ui/native_theme/native_theme_base.h

Index: ui/native_theme/native_theme_base.h
diff --git a/ui/native_theme/native_theme_base.h b/ui/native_theme/native_theme_base.h
index a8677d3bf30e1cb574fee45492fc88f52f4b1422..4e9ffaea4409c2ec0f56856eed630efdb5fe2a26 100644
--- a/ui/native_theme/native_theme_base.h
+++ b/ui/native_theme/native_theme_base.h
@@ -142,6 +142,11 @@ class NATIVE_THEME_EXPORT NativeThemeBase : public NativeTheme {
                                                   const gfx::Rect& rect) const {
   }
 
+  virtual void PaintComboboxArrowBackground(SkCanvas* canvas,
+                                            State state,
+                                            const gfx::Rect& rect) const {
+  }
+
   // Shrinks checkbox/radio button rect, if necessary, to make room for padding
   // and drop shadow.
   // TODO(mohsen): This is needed because checkboxes/radio buttons on Android