Mac: Don't repaint scrollbars every frame

cc/blink/scrollbar_impl.cc

Index: cc/blink/scrollbar_impl.cc
diff --git a/cc/blink/scrollbar_impl.cc b/cc/blink/scrollbar_impl.cc
index 7efd51375666187b9889a16bf0b9bf00a7ef565e..6f5fe31d12de71d09f44cd1c65eb37d5e1999fae 100644
--- a/cc/blink/scrollbar_impl.cc
+++ b/cc/blink/scrollbar_impl.cc
@@ -63,6 +63,16 @@ gfx::Rect ScrollbarImpl::TrackRect() const {
   return geometry_->trackRect(scrollbar_.get());
 }
 
+float ScrollbarImpl::ThumbOpacity() const {
+  return painter_.thumbOpacity();
+}
+
+bool ScrollbarImpl::NeedsPaintPart(cc::ScrollbarPart part) const {
+  if (part == cc::THUMB)
+    return painter_.thumbNeedsRepaint();
+  return painter_.trackNeedsRepaint();
+}
+
 void ScrollbarImpl::PaintPart(SkCanvas* canvas,
                               cc::ScrollbarPart part,
                               const gfx::Rect& content_rect) {