[heap] Fix finalization of incremental marking race.

Between requesting finalization of incremental marking and the time where we handle the request in the stack guard, the current full GC may have finished. In that case the stack guard triggers to late and tries to finalize marking in a state where marking is not going on.

Note that a cleaner fix would be to express the finalization phase in a special marking phase. I will do that in a follow-up CL.

BUG=

Review URL: https://codereview.chromium.org/1493133003

Cr-Commit-Position: refs/heads/master@{#32552}

src/heap/heap.cc

@@ -764,7 +764,8 @@ void Heap::HandleGCRequest() {
     return;
   }
   DCHECK(FLAG_finalize_marking_incrementally);
-  if (!incremental_marking()->finalize_marking_completed()) {
+  if (incremental_marking()->IsMarking() &&
+      !incremental_marking()->finalize_marking_completed()) {
     FinalizeIncrementalMarking("GC interrupt: finalize incremental marking");
   }
 }