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 e7c21b8ad1f11a6feb80c19c6fa2e0c6b5da1bbc..91a69cb9b32e9e197ff78245387966c48b734d6d 100644
--- a/cc/blink/scrollbar_impl.cc
+++ b/cc/blink/scrollbar_impl.cc
@@ -64,6 +64,20 @@ gfx::Rect ScrollbarImpl::TrackRect() const {
   return geometry_->trackRect(scrollbar_.get());
 }
 
+float ScrollbarImpl::ThumbOpacity() const {
+  return painter_.thumbOpacity();
+}
+
+float ScrollbarImpl::TrackOpacity() const {
+  return painter_.trackOpacity();
+}
+
+bool ScrollbarImpl::NeedsPaintPart(cc::ScrollbarPart part) const {
+  if (part == cc::THUMB)

[enne (OOO), 2015/11/23 19:05:24]

This should just return false and maybe also NOTREACHED, as it doesn't apply on
the compositor thread?

[ccameron, 2015/11/24 01:12:04]

Mmh, this is called on the main thread -- the "Impl" in ScrollbarImpl is "the
implementation of the abstract class Scrollbar", not "on the impl thread".

[enne (OOO), 2015/11/24 17:59:18]

<_<

+    return painter_.needsPaintThumb();
+  return painter_.needsPaintTrack();
+}
+
 void ScrollbarImpl::PaintPart(SkCanvas* canvas,
                               cc::ScrollbarPart part,
                               const gfx::Rect& content_rect) {