Fix tracing of element rare data

ElementRareData is already traced in Node::Trace(), so there's no need
to double-trace it in Element::Trace().

By removing the conditional tracing that depends on node flags, this
also fixes a data race between main thread and GC thread, which should
not check node flags.

Bug: 1056233
Change-Id: If51f7946c5c41240cdc28969fb6eaad8891ff66e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083817Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746178}

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

@@ -6405,8 +6405,6 @@ void Element::LogUpdateAttributeIfIsolatedWorldAndInDocument(
 }
 
 void Element::Trace(Visitor* visitor) {
-  if (HasRareData())
-    visitor->Trace(GetElementRareData());
   visitor->Trace(element_data_);
   ContainerNode::Trace(visitor);
 }