[heap] Fix cctest/test-concurrent-marking/ConcurrentMarking

BUG=chromium:694255

Change-Id: Ib954eaf868862577fb567a1438ca1c9136821eb4
Reviewed-on: https://chromium-review.googlesource.com/549417Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46251}

test/cctest/heap/test-concurrent-marking.cc

@@ -23,6 +23,8 @@ TEST(ConcurrentMarking) {
   for (int i = 0; i <= Worklist::kSegmentCapacity; i++) {
     shared.Push(0, heap->undefined_value());
   }
+  HeapObject* object;
+  CHECK(shared.Pop(0, &object));
   ConcurrentMarking* concurrent_marking =
       new ConcurrentMarking(heap, &shared, &bailout);
   concurrent_marking->StartTask();