Mac: Don't repaint scrollbars every frame

cc/blink/scrollbar_impl.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/blink/scrollbar_impl.h"
 
 #include "base/logging.h"
 #include "third_party/WebKit/public/platform/WebScrollbar.h"
 #include "third_party/WebKit/public/platform/WebScrollbarThemeGeometry.h"
 
@@ skipping 45 matching lines @@
   gfx::Rect thumb_rect = geometry_->thumbRect(scrollbar_.get());
   if (scrollbar_->orientation() == WebScrollbar::Horizontal)
     return thumb_rect.width();
   return thumb_rect.height();
 }
 
 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) {
   if (part == cc::THUMB) {
     painter_.paintThumb(canvas, content_rect);
     return;
   }
 
   // The following is a simplification of ScrollbarThemeComposite::paint.
   painter_.paintScrollbarBackground(canvas, content_rect);
@@ skipping 22 matching lines @@
   bool thumb_present = geometry_->hasThumb(scrollbar_.get());
   if (thumb_present) {
     painter_.paintForwardTrackPart(canvas, track_paint_rect);
     painter_.paintBackTrackPart(canvas, track_paint_rect);
   }
 
   painter_.paintTickmarks(canvas, track_paint_rect);
 }
 
 }  // namespace cc_blink