Add map from element id to scroll node index.

cc/trees/layer_tree_host_impl.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_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 3923 matching lines @@
     return;
 
   const int layer_id = tree->LayerIdByElementId(element_id);
   PropertyTrees* property_trees = tree->property_trees();
   DCHECK(property_trees->IsInIdToIndexMap(PropertyTrees::TreeType::SCROLL,
                                           layer_id));
   DCHECK_EQ(
       1u, property_trees->element_id_to_transform_node_index.count(element_id));
   int transform_node_index =
       property_trees->element_id_to_transform_node_index[element_id];
-  // TODO(wkorman): Build map from element id to scroll node in property tree
-  // builder and make use of it below.
+  DCHECK_EQ(1u,
+            property_trees->element_id_to_scroll_node_index.count(element_id));
   const int scroll_node_index =
-      property_trees->layer_id_to_scroll_node_index[layer_id];
-  if (scroll_node_index != ScrollTree::kInvalidNodeId) {
-    property_trees->scroll_tree.OnScrollOffsetAnimated(
-        layer_id, transform_node_index, scroll_node_index, scroll_offset, tree);
-    // Run mutation callbacks to respond to updated scroll offset.
-    Mutate(CurrentBeginFrameArgs().frame_time);
-  }
+      property_trees->element_id_to_scroll_node_index[element_id];
+  property_trees->scroll_tree.OnScrollOffsetAnimated(
+      layer_id, transform_node_index, scroll_node_index, scroll_offset, tree);
+  // Run mutation callbacks to respond to updated scroll offset.
+  Mutate(CurrentBeginFrameArgs().frame_time);
 }
 
 bool LayerTreeHostImpl::AnimationsPreserveAxisAlignment(
     const LayerImpl* layer) const {
   return mutator_host_->AnimationsPreserveAxisAlignment(layer->element_id());
 }
 
 void LayerTreeHostImpl::SetNeedUpdateGpuRasterizationStatus() {
   need_update_gpu_rasterization_status_ = true;
 }
@@ skipping 116 matching lines @@
       worker_context_visibility_ =
           worker_context->CacheController()->ClientBecameVisible();
     } else {
       worker_context->CacheController()->ClientBecameNotVisible(
           std::move(worker_context_visibility_));
     }
   }
 }
 
 }  // namespace cc