Track the currently scrolling ScrollNode instead of the scrolling layer

cc/trees/layer_tree_host_common.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/trees/layer_tree_host_common.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 586 matching lines @@
   CalculateDrawPropertiesInternal(inputs, DONT_BUILD_PROPERTY_TREES);
 
   if (CdpPerfTracingEnabled()) {
     LayerTreeImpl* layer_tree_impl = inputs->root_layer->layer_tree_impl();
     if (layer_tree_impl->IsPendingTree() &&
         layer_tree_impl->is_first_frame_after_commit()) {
       LayerImpl* active_tree_root =
           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,
             // it is enough to calculate jitter on one of these layers. So,
             // 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) {
                 jitter_nodes.insert(layer->scroll_tree_index());
                 jitter += layer_jitter;
               }
             }
           }
         }
       }
@@ skipping 20 matching lines @@
 
 PropertyTrees* GetPropertyTrees(Layer* layer) {
   return layer->layer_tree_host()->property_trees();
 }
 
 PropertyTrees* GetPropertyTrees(LayerImpl* layer) {
   return layer->layer_tree_impl()->property_trees();
 }
 
 }  // namespace cc