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

cc/layer_tree_impl.cc

 // Copyright 2011 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/layer_tree_impl.h"
 
 #include "base/debug/trace_event.h"
 #include "cc/layer_tree_host_common.h"
 #include "cc/layer_tree_host_impl.h"
 #include "ui/gfx/vector2d_conversions.h"
@@ skipping 98 matching lines @@
   max_scroll.ClampToMin(gfx::Vector2dF());
 
   root_scroll_layer()->setMaxScrollOffset(gfx::ToFlooredVector2d(max_scroll));
 }
 
 void LayerTreeImpl::UpdateDrawProperties() {
   render_surface_layer_list_.clear();
   if (!RootLayer())
     return;
 
-  if (root_scroll_layer()) {
-    root_scroll_layer()->setImplTransform(
-        layer_tree_host_impl_->implTransform());
+  for (size_t i = 0; i < RootLayer()->children().size(); ++i) {
+    LayerImpl* layerImpl = RootLayer()->children()[i];
+    layerImpl->setImplTransform(layer_tree_host_impl_->implTransform());

[Ian Vollick, 2013/01/15 20:33:48]

Please add a comment here that the implTransform is a generalization of page
scale for pinch zoom that includes translation, and that pinch zoom is, by
design, supposed to affect the scale of the main frame's scrollbars so we need
to apply it to all descendants. It's also worth noting here that you can't just
apply it to the root since the root makes assumptions about never being
translated.

   }
 
   {
     TRACE_EVENT0("cc", "LayerTreeImpl::UpdateDrawProperties");
     LayerTreeHostCommon::calculateDrawProperties(
         RootLayer(),
         device_viewport_size(),
         device_scale_factor(),
         pinch_zoom_viewport().page_scale_factor(),
         MaxTextureSize(),
@@ skipping 135 matching lines @@
 
 AnimationRegistrar* LayerTreeImpl::animationRegistrar() const {
   return layer_tree_host_impl_->animationRegistrar();
 }
 
 const PinchZoomViewport& LayerTreeImpl::pinch_zoom_viewport() const {
   return layer_tree_host_impl_->pinchZoomViewport();
 }
 
 } // namespace cc