heap: Resolve data race in heap.cc DCHECK

This check verified that either concurrent marking was enabled, or
the worklist for keeping v8 references found by concurrent markers
is empty.
Checking that the worklist is empty when concurrent markers are
active results in a data race. Switching the order of the conditions
in the DCHECK means will only check the worklist if we don't have any
concurrent markers.

Bug: 986235
Change-Id: I0c4b15c944c3d52dc72fb150b47b97b03d475f17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1892772
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711487}

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

@@ -548,9 +548,9 @@ void ThreadHeap::FlushV8References() {
   if (!thread_state_->IsUnifiedGCMarkingInProgress())
     return;
 
-  DCHECK(v8_references_worklist_->IsGlobalEmpty() ||
-         base::FeatureList::IsEnabled(
-             blink::features::kBlinkHeapConcurrentMarking));
+  DCHECK(base::FeatureList::IsEnabled(
+             blink::features::kBlinkHeapConcurrentMarking) ||
+         v8_references_worklist_->IsGlobalEmpty());
 
   V8ReferencesWorklist::View v8_references(v8_references_worklist_.get(),
                                            WorklistTaskId::MutatorThread);