[heap] Fix benign race in DCHECKs for mark bits

During minor MC new space evacuation we could have two adjacent LABs
sharing a mark bit cell. The DCHECK when observing and changing markbits
of the target needs to reflect that.

Bug: chromium:651354
Change-Id: I737d0f9e3d37dfb1cda3f126d37ed5e7123bedc9
Reviewed-on: https://chromium-review.googlesource.com/541296Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46032}

src/heap/mark-compact.cc

@@ -1585,7 +1585,7 @@ class YoungGenerationMigrationObserver final : public MigrationObserver {
     // Migrate color to old generation marking in case the object survived young
     // generation garbage collection.
     if (heap_->incremental_marking()->IsMarking()) {
-      DCHECK(ObjectMarking::IsWhite(
+      DCHECK(ObjectMarking::IsWhite<AccessMode::ATOMIC>(
           dst, mark_compact_collector_->marking_state(dst)));
       heap_->incremental_marking()->TransferColor<AccessMode::ATOMIC>(src, dst);
     }

src/heap/marking.h

@@ -237,7 +237,7 @@ class Marking : public AllStatic {
   static const char* kWhiteBitPattern;
   template <AccessMode mode = AccessMode::NON_ATOMIC>
   INLINE(static bool IsWhite(MarkBit mark_bit)) {
-    DCHECK(!IsImpossible(mark_bit));
+    DCHECK(!IsImpossible<mode>(mark_bit));
     return !mark_bit.Get<mode>();
   }