MacViews: Implement scrollbar theming.

ui/native_theme/native_theme_mac.h

Index: ui/native_theme/native_theme_mac.h
diff --git a/ui/native_theme/native_theme_mac.h b/ui/native_theme/native_theme_mac.h
index 0b15b3f50950e263e16257ae81c28f8fd1fc341b..ecdd38ebacb73e20a844e441b943675b37bda47e 100644
--- a/ui/native_theme/native_theme_mac.h
+++ b/ui/native_theme/native_theme_mac.h
@@ -20,6 +20,19 @@ class NativeThemeMac : public FallbackTheme {
   virtual SkColor GetSystemColor(ColorId color_id) const OVERRIDE;
 
   // Overridden from NativeThemeBase:
+  virtual void PaintScrollbarTrack(
+      SkCanvas* canvas,
+      Part part,
+      State state,
+      const ScrollbarTrackExtraParams& extra_params,
+      const gfx::Rect& rect) const OVERRIDE;
+  virtual void PaintScrollbarThumb(SkCanvas* sk_canvas,
+                                   Part part,
+                                   State state,
+                                   const gfx::Rect& rect) const OVERRIDE;
+  virtual void PaintScrollbarCorner(SkCanvas* canvas,
+                                    State state,
+                                    const gfx::Rect& rect) const OVERRIDE;
   virtual void PaintMenuPopupBackground(
       SkCanvas* canvas,
       const gfx::Size& size,