aura: Let NativeThemeBase paint ArrowButtons for aura.

ui/base/native_theme/native_theme_aura.cc

Index: ui/base/native_theme/native_theme_aura.cc
diff --git a/ui/base/native_theme/native_theme_aura.cc b/ui/base/native_theme/native_theme_aura.cc
index f6328084896a116657577923f35c117f798b7d82..b918a7168c9a72e5535bb93448c3dacc838f045d 100644
--- a/ui/base/native_theme/native_theme_aura.cc
+++ b/ui/base/native_theme/native_theme_aura.cc
@@ -227,14 +227,6 @@ void NativeThemeAura::PaintScrollbarTrack(
   }
 }
 
-void NativeThemeAura::PaintArrowButton(SkCanvas* canvas,
-                                       const gfx::Rect& rect,
-                                       Part part,
-                                       State state) const {
-  // TODO(jamescook): Should this paint something?  We used to DCHECK() here
-  // that the rect was empty, but that was failing on about: UI pages.
-}
-
 void NativeThemeAura::PaintScrollbarThumb(SkCanvas* canvas,
                                           Part part,
                                           State state,