cc: Preserve currently scrolling layer during tree activation

cc/layer_tree_impl.cc

Index: cc/layer_tree_impl.cc
diff --git a/cc/layer_tree_impl.cc b/cc/layer_tree_impl.cc
index e07fa03d2b0c43b7fbfa2ee2e2c119a55c2059c5..0f818e967d302a3ebf2e47952ef759658ee79f42 100644
--- a/cc/layer_tree_impl.cc
+++ b/cc/layer_tree_impl.cc
@@ -177,6 +177,12 @@ void LayerTreeImpl::UnregisterLayer(LayerImpl* layer) {
   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();
 }