[heap] MinorMC: Fix race when flushing back live bytes

NOTRY=true
TBR=hpayer@chromium.org

Bug: chromium:651354
Change-Id: Ibff438d987b07907b2c4463a42bef685864c1602
Reviewed-on: https://chromium-review.googlesource.com/513903Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45505}

src/heap/mark-compact.cc

@@ -2335,8 +2335,6 @@ class YoungGenerationMarkingTask : public ItemParallelJob::Task {
                               Page::kPageSize);
   }
 
-  ~YoungGenerationMarkingTask() { FlushLiveBytes(); }
-
   void RunInParallel() override {
     double marking_time = 0.0;
     {
@@ -2349,6 +2347,7 @@ class YoungGenerationMarkingTask : public ItemParallelJob::Task {
       }
       EmptyMarkingDeque();
       DCHECK(marking_deque_.IsEmpty());
+      FlushLiveBytes();
     }
     if (FLAG_trace_minor_mc_parallel_marking) {
       PrintIsolate(collector_->isolate(), "marking[%p]: time=%f\n",