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

cc/trees/tree_synchronizer_unittest.cc

Index: cc/trees/tree_synchronizer_unittest.cc
diff --git a/cc/trees/tree_synchronizer_unittest.cc b/cc/trees/tree_synchronizer_unittest.cc
index 11b68ad4e779225e128e47beb7d600d494e0b1fe..cbae4e4aea1ba7de93a2f9b7ebe95a587f99a1d5 100644
--- a/cc/trees/tree_synchronizer_unittest.cc
+++ b/cc/trees/tree_synchronizer_unittest.cc
@@ -493,6 +493,9 @@ TEST_F(TreeSynchronizerTest, SynchronizeCurrentlyScrollingNode) {
   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());
@@ -537,6 +540,9 @@ TEST_F(TreeSynchronizerTest, SynchronizeScrollTreeScrollOffsetMap) {
   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);