Track the currently scrolling ScrollNode instead of the scrolling layer

cc/trees/layer_tree_host_common.cc

Index: cc/trees/layer_tree_host_common.cc
diff --git a/cc/trees/layer_tree_host_common.cc b/cc/trees/layer_tree_host_common.cc
index 205b95f732a056203fa78efe8b3b4bdb029b694a..85886b6071a0487ad2fed84ca5c496857b626669 100644
--- a/cc/trees/layer_tree_host_common.cc
+++ b/cc/trees/layer_tree_host_common.cc
@@ -604,11 +604,9 @@ void LayerTreeHostCommon::CalculateDrawProperties(
           layer_tree_impl->FindActiveTreeLayerById(inputs->root_layer->id());
       float jitter = 0.f;
       if (active_tree_root) {
-        LayerImpl* last_scrolled_layer = layer_tree_impl->LayerById(
-            active_tree_root->layer_tree_impl()->LastScrolledLayerId());
-        if (last_scrolled_layer) {
-          const int last_scrolled_scroll_node_id =
-              last_scrolled_layer->scroll_tree_index();
+        int last_scrolled_node_index =
+            active_tree_root->layer_tree_impl()->LastScrolledScrollNodeIndex();
+        if (last_scrolled_node_index) {

[ajuma, 2017/02/28 22:59:37]

This needs to be compared to ScrollTree::kInvalidNodeId

[pdr., 2017/03/01 07:21:03]

Done

           std::unordered_set<int> jitter_nodes;
           for (auto* layer : *layer_tree_impl) {
             // Layers that have the same scroll tree index jitter together. So,
@@ -616,7 +614,7 @@ void LayerTreeHostCommon::CalculateDrawProperties(
             // after we find a jittering layer, we need not consider other
             // layers with the same scroll tree index.
             int scroll_tree_index = layer->scroll_tree_index();
-            if (last_scrolled_scroll_node_id <= scroll_tree_index &&
+            if (last_scrolled_node_index <= scroll_tree_index &&
                 jitter_nodes.find(scroll_tree_index) == jitter_nodes.end()) {
               float layer_jitter = CalculateLayerJitter(layer);
               if (layer_jitter > 0.f) {