heap: Check TracedReference::IsEmpty concurrently

This CL is dependent on
https://chromium-review.googlesource.com/c/v8/v8/+/2327910/

Bug: 1108537
Change-Id: Ie2121ba8e1460a5979ceeba734ca3a516a63903f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2327672
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794160}

third_party/blink/renderer/platform/bindings/trace_wrapper_v8_reference.h

@@ -40,6 +40,7 @@ class TraceWrapperV8Reference {
   }
 
   bool IsEmpty() const { return handle_.IsEmpty(); }
+  bool IsEmptySafe() const { return handle_.IsEmptyThreadSafe(); }
   void Clear() { handle_.Reset(); }
   ALWAYS_INLINE const v8::TracedReference<T>& Get() const { return handle_; }
   ALWAYS_INLINE v8::TracedReference<T>& Get() { return handle_; }

third_party/blink/renderer/platform/heap/unified_heap_marking_visitor.cc

@@ -29,6 +29,8 @@ UnifiedHeapMarkingVisitorBase::UnifiedHeapMarkingVisitorBase(
 void UnifiedHeapMarkingVisitorBase::VisitImpl(
     const TraceWrapperV8Reference<v8::Value>& v8_reference) {
   DCHECK(isolate_);
+  if (v8_reference.IsEmptySafe())
+    return;
   if (task_id_ != WorklistTaskId::MutatorThread) {
     // This is a temporary solution. Pushing directly from concurrent threads
     // to V8 marking worklist will currently result in data races. This
@@ -38,8 +40,6 @@ void UnifiedHeapMarkingVisitorBase::VisitImpl(
     v8_references_worklist_.Push(&v8_reference);
     return;
   }
-  if (v8_reference.Get().IsEmpty())
-    return;
   controller_->RegisterEmbedderReference(
       v8_reference.template Cast<v8::Data>().Get());
 }