Implement pinch-zoom scaling for main-frame scrollbars and pinch-zoom overlay scrollbars.

cc/scrollbar_layer.cc

 // Copyright 2012 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/scrollbar_layer.h"
 
 #include "base/basictypes.h"
 #include "base/debug/trace_event.h"
 #include "cc/caching_bitmap_content_layer_updater.h"
 #include "cc/layer_painter.h"
@@ skipping 43 matching lines @@
 ScrollbarLayer::~ScrollbarLayer() {}
 
 void ScrollbarLayer::SetScrollLayerId(int id) {
   if (id == scroll_layer_id_)
     return;
 
   scroll_layer_id_ = id;
   SetNeedsFullTreeSync();
 }
 
+bool ScrollbarLayer::DoNotSkipOnZeroOpacity() const {
+  return scroll_layer_id_ == Layer::PINCH_ZOOM_ROOT_SCROLL_LAYER_ID;
+}
+
 WebKit::WebScrollbar::Orientation ScrollbarLayer::Orientation() const {
   return scrollbar_->orientation();
 }
 
 int ScrollbarLayer::MaxTextureSize() {
   DCHECK(layer_tree_host());
   return layer_tree_host()->rendererCapabilities().maxTextureSize;
 }
 
 float ScrollbarLayer::ClampScaleToMaxTextureSize(float scale) {
@@ skipping 348 matching lines @@
                thumb_.get(),
                origin_thumb_rect,
                queue,
                stats);
   }
 
   dirty_rect_ = gfx::RectF();
 }
 
 }  // namespace cc