[heap] Fix a race introduced in 2e70adc7

The marked bytes counter needs to be updated before decrementing the
pending task counter.

Bug: chromium:694255
Change-Id: I19c4dfbdccfb32ded5b7bb707dc93d53e188e34a
Reviewed-on: https://chromium-review.googlesource.com/707140Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48390}

src/heap/concurrent-marking.cc

@@ -398,14 +398,14 @@ void ConcurrentMarking::Run(int task_id, TaskState* task_state) {
     }
     weak_objects_->weak_cells.FlushToGlobal(task_id);
     weak_objects_->transition_arrays.FlushToGlobal(task_id);
+    base::AsAtomicWord::Relaxed_Store<size_t>(&task_state->marked_bytes, 0);
+    total_marked_bytes_.Increment(marked_bytes);
     {
       base::LockGuard<base::Mutex> guard(&pending_lock_);
       is_pending_[task_id] = false;
       --pending_task_count_;
       pending_condition_.NotifyAll();
     }
-    base::AsAtomicWord::Relaxed_Store<size_t>(&task_state->marked_bytes, 0);
-    total_marked_bytes_.Increment(marked_bytes);
   }
   if (FLAG_trace_concurrent_marking) {
     heap_->isolate()->PrintWithTimestamp(