Refactor delegated scrolling in LayerImpl

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 2c13fcbc861f58e298a6a9c073f3eca33c4574f4..6bdd0eb4b15fb34c786fb3d6687331e673a173ca 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -3019,7 +3019,14 @@ static void CollectScrollDeltas(ScrollAndScaleSet* scroll_info,
 scoped_ptr<ScrollAndScaleSet> LayerTreeHostImpl::ProcessScrollDeltas() {
   scoped_ptr<ScrollAndScaleSet> scroll_info(new ScrollAndScaleSet());
 
+  if (root_layer_scroll_offset_delegate_)
+    root_layer_scroll_offset_delegate_->SetBlockUpdates(true);
   CollectScrollDeltas(scroll_info.get(), active_tree_->root_layer());
+  if (root_layer_scroll_offset_delegate_)
+    root_layer_scroll_offset_delegate_->SetBlockUpdates(false);
+
+  active_tree_->UpdateScrollOffsetDelegateIfNeeded();
+
   scroll_info->page_scale_delta =
       active_tree_->page_scale_factor()->PullDeltaForMainThread();
   scroll_info->top_controls_delta =