Use element ids as a stable identifier tracking scroll nodes across updates

cc/trees/tree_synchronizer_unittest.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/tree_synchronizer.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <set>
@@ skipping 475 matching lines @@
   scoped_refptr<Layer> scroll_clip_layer = Layer::Create();
   scoped_refptr<Layer> scroll_layer = Layer::Create();
   scoped_refptr<Layer> transient_scroll_clip_layer = Layer::Create();
   scoped_refptr<Layer> transient_scroll_layer = Layer::Create();
 
   layer_tree_root->AddChild(transient_scroll_clip_layer);
   transient_scroll_clip_layer->AddChild(transient_scroll_layer);
   transient_scroll_layer->AddChild(scroll_clip_layer);
   scroll_clip_layer->AddChild(scroll_layer);
 
+  ElementId scroll_element_id = ElementId(5, 4);
+  scroll_layer->SetElementId(scroll_element_id);
+
   transient_scroll_layer->SetScrollClipLayerId(
       transient_scroll_clip_layer->id());
   scroll_layer->SetScrollClipLayerId(scroll_clip_layer->id());
   host_->SetRootLayer(layer_tree_root);
   host_->BuildPropertyTreesForTesting();
   host_->CommitAndCreatePendingTree();
   host_impl->ActivateSyncTree();
 
   ExpectTreesAreIdentical(layer_tree_root.get(),
                           host_impl->active_tree()->root_layer_for_testing(),
@@ skipping 24 matching lines @@
   TestTaskGraphRunner task_graph_runner;
   FakeLayerTreeHostImpl* host_impl = host_->host_impl();
   host_impl->CreatePendingTree();
 
   scoped_refptr<Layer> layer_tree_root = Layer::Create();
   scoped_refptr<Layer> scroll_clip_layer = Layer::Create();
   scoped_refptr<Layer> scroll_layer = Layer::Create();
   scoped_refptr<Layer> transient_scroll_clip_layer = Layer::Create();
   scoped_refptr<Layer> transient_scroll_layer = Layer::Create();
 
+  ElementId scroll_element_id = ElementId(5, 4);
+  scroll_layer->SetElementId(scroll_element_id);
+
   layer_tree_root->AddChild(transient_scroll_clip_layer);
   transient_scroll_clip_layer->AddChild(transient_scroll_layer);
   transient_scroll_layer->AddChild(scroll_clip_layer);
   scroll_clip_layer->AddChild(scroll_layer);
 
   transient_scroll_layer->SetScrollClipLayerId(
       transient_scroll_clip_layer->id());
   scroll_layer->SetScrollClipLayerId(scroll_clip_layer->id());
   transient_scroll_layer->SetScrollOffset(gfx::ScrollOffset(1, 2));
   scroll_layer->SetScrollOffset(gfx::ScrollOffset(10, 20));
@@ skipping 119 matching lines @@
   host_->CommitAndCreatePendingTree();
   host_impl->ActivateSyncTree();
   EXPECT_EQ(
       CombinedAnimationScale(0.f, 0.f),
       host_impl->active_tree()->property_trees()->GetAnimationScales(
           transform_layer->transform_tree_index(), host_impl->active_tree()));
 }
 
 }  // namespace
 }  // namespace cc