Use the default theme as the fallback UI in Blink

ui/native_theme/native_theme_aura.h

Index: ui/native_theme/native_theme_aura.h
diff --git a/ui/native_theme/native_theme_aura.h b/ui/native_theme/native_theme_aura.h
index 51ebd383c7127a58bca96b6782a4a9593c59b9bf..b3417b353ced7af13b035872b3515a5cfc875c80 100644
--- a/ui/native_theme/native_theme_aura.h
+++ b/ui/native_theme/native_theme_aura.h
@@ -7,12 +7,12 @@
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
-#include "ui/native_theme/native_theme_base.h"
+#include "ui/native_theme/fallback_theme.h"
 
 namespace ui {
 
 // Aura implementation of native theme support.
-class NATIVE_THEME_EXPORT NativeThemeAura : public NativeThemeBase {
+class NATIVE_THEME_EXPORT NativeThemeAura : public FallbackTheme {
  public:
   static NativeThemeAura* instance();
 
@@ -21,7 +21,6 @@ class NATIVE_THEME_EXPORT NativeThemeAura : public NativeThemeBase {
   virtual ~NativeThemeAura();
 
   // Overridden from NativeThemeBase:
-  virtual SkColor GetSystemColor(ColorId color_id) const OVERRIDE;
   virtual void PaintMenuPopupBackground(
       SkCanvas* canvas,
       const gfx::Size& size,