[heap] Fix data race in IncrementalMarking::NotifyLeftTrimming.

BUG=chromium:694255
TBR=mlippautz@chromium.org

Change-Id: I7dd9623ff85fcc49f034c71a6f5149f9488a9abb
Reviewed-on: https://chromium-review.googlesource.com/593010Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46990}

src/heap/incremental-marking.cc

@@ -141,7 +141,7 @@ void IncrementalMarking::NotifyLeftTrimming(HeapObject* from, HeapObject* to) {
   MarkBit old_mark_bit = ObjectMarking::MarkBitFrom(from, marking_state(from));
   MarkBit new_mark_bit = ObjectMarking::MarkBitFrom(to, marking_state(to));
 
-  if (black_allocation() && Marking::IsBlack(new_mark_bit)) {
+  if (black_allocation() && Marking::IsBlack<kAtomicity>(new_mark_bit)) {
     // Nothing to do if the object is in black area.
     return;
   }