[heap] Unconditionally mark objects that undergo unsafe layout changes.

In the upcoming concurrent marking protocol the main thread will have to
mark objects before performing unsafe layout change.

This patch is an experiment for evaluating memory the impact of marking
an object on layout change.

BUG=chromium:694255

Review-Url: https://codereview.chromium.org/2706213002
Cr-Commit-Position: refs/heads/master@{#43351}

src/heap/heap.cc

@@ -4304,7 +4304,9 @@ void Heap::RegisterReservationsForBlackAllocation(Reservation* reservations) {
 
 void Heap::NotifyObjectLayoutChange(HeapObject* object,
                                     const DisallowHeapAllocation&) {
-// TODO(ulan): Add synchronization with the concurrent marker.
+  if (FLAG_incremental_marking && incremental_marking()->IsMarking()) {
+    incremental_marking()->MarkGrey(this, object);
+  }
 #ifdef VERIFY_HEAP
   DCHECK(pending_layout_change_object_ == nullptr);
   pending_layout_change_object_ = object;