cc : Calculate jitter without using layer hierarchy

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 0420344247132d31c1a144f60545119b769a9d73..2cc0be085c09e784a47b8c6bdcf09b961870e09d 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -61,7 +61,7 @@ LayerTreeImpl::LayerTreeImpl(
       hud_layer_(nullptr),
       background_color_(0),
       has_transparent_background_(false),
-      last_scrolled_layer_id_(Layer::INVALID_ID),
+      last_scrolled_scroll_node_id_(Layer::INVALID_ID),
       overscroll_elasticity_layer_id_(Layer::INVALID_ID),
       page_scale_layer_id_(Layer::INVALID_ID),
       inner_viewport_scroll_layer_id_(Layer::INVALID_ID),
@@ -515,8 +515,8 @@ LayerImpl* LayerTreeImpl::CurrentlyScrollingLayer() const {
   return LayerById(scroll_node ? scroll_node->owner_id : Layer::INVALID_ID);
 }
 
-int LayerTreeImpl::LastScrolledLayerId() const {
-  return last_scrolled_layer_id_;
+int LayerTreeImpl::LastScrolledScrollNodeId() const {
+  return last_scrolled_scroll_node_id_;
 }
 
 void LayerTreeImpl::SetCurrentlyScrollingLayer(LayerImpl* layer) {
@@ -526,7 +526,7 @@ void LayerTreeImpl::SetCurrentlyScrollingLayer(LayerImpl* layer) {
   int new_id = layer ? layer->id() : Layer::INVALID_ID;
   int new_scroll_node_id = layer ? layer->scroll_tree_index() : -1;
   if (layer)
-    last_scrolled_layer_id_ = new_id;
+    last_scrolled_scroll_node_id_ = new_scroll_node_id;
 
   if (old_id == new_id)
     return;