cc: Preserve currently scrolling layer during tree activation

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 159 matching lines @@
 void LayerTreeImpl::RegisterLayer(LayerImpl* layer) {
   DCHECK(!LayerById(layer->id()));
   layer_id_map_[layer->id()] = layer;
 }
 
 void LayerTreeImpl::UnregisterLayer(LayerImpl* layer) {
   DCHECK(LayerById(layer->id()));
   layer_id_map_.erase(layer->id());
 }
 
+void LayerTreeImpl::PushPersistedState(LayerTreeImpl* pendingTree) {
+  int id = currently_scrolling_layer_ ? currently_scrolling_layer_->id() : 0;
+  pendingTree->set_currently_scrolling_layer(
+      LayerTreeHostCommon::findLayerInSubtree(pendingTree->RootLayer(), id));
+}
+
 const LayerTreeSettings& LayerTreeImpl::settings() const {
   return layer_tree_host_impl_->settings();
 }
 
 OutputSurface* LayerTreeImpl::output_surface() const {
   return layer_tree_host_impl_->outputSurface();
 }
 
 ResourceProvider* LayerTreeImpl::resource_provider() const {
   return layer_tree_host_impl_->resourceProvider();
@@ skipping 67 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