Fix GC issue with Element::part.

ElementRareDate::TraceAfterDispatch did not trace the part_ field. The fix is to
make it trace it.

Bug: 909609
Change-Id: I8fd00e7613af69272ff12298b0ea33d6494f80e5
Reviewed-on: https://chromium-review.googlesource.com/c/1354740Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612050}

third_party/blink/renderer/core/dom/element_rare_data.cc

@@ -108,6 +108,7 @@ ElementInternals& ElementRareData::EnsureElementInternals(HTMLElement& target) {
 void ElementRareData::TraceAfterDispatch(blink::Visitor* visitor) {
   visitor->Trace(dataset_);
   visitor->Trace(class_list_);
+  visitor->Trace(part_);
   visitor->Trace(shadow_root_);
   visitor->Trace(attribute_map_);
   visitor->Trace(attr_node_list_);