Change cc::ElementId to be a uint64_t

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 7abf735a319f48b7bca8608574a9873852d33fce..f71ad7d8c795e9046d6c6d8697dd24d3f27ccb0c 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -1211,7 +1211,7 @@ void LayerTreeHost::PushPropertiesTo(LayerTreeImpl* tree_impl) {
 }
 
 Layer* LayerTreeHost::LayerByElementId(ElementId element_id) const {
-  auto iter = element_layers_map_.find(element_id);
+  auto iter = element_layers_map_.find(element_id.id);
   return iter != element_layers_map_.end() ? iter->second : nullptr;
 }
 
@@ -1219,7 +1219,7 @@ void LayerTreeHost::RegisterElement(ElementId element_id,
                                     ElementListType list_type,
                                     Layer* layer) {
   if (layer->element_id()) {
-    element_layers_map_[layer->element_id()] = layer;
+    element_layers_map_[layer->element_id().id] = layer;
   }
 
   mutator_host_->RegisterElement(element_id, list_type);
@@ -1231,12 +1231,14 @@ void LayerTreeHost::UnregisterElement(ElementId element_id,
   mutator_host_->UnregisterElement(element_id, list_type);
 
   if (layer->element_id()) {
-    element_layers_map_.erase(layer->element_id());
+    element_layers_map_.erase(layer->element_id().id);
   }
 }
 
 static void SetElementIdForTesting(Layer* layer) {
-  layer->SetElementId(LayerIdToElementIdForTesting(layer->id()));
+  ElementId element_id;
+  element_id.id = layer->id();
+  layer->SetElementId(element_id);
 }
 
 void LayerTreeHost::SetElementIdsForTesting() {