[heap] Fix ConcurrentMarkingReschedule cctest.

This makes the test robust for stress GC flags.

Bug: 
Change-Id: Ica65987f0ee09fbdb4aab233dea4c51db5b19459
Reviewed-on: https://chromium-review.googlesource.com/702436Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48313}

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

@@ -31,6 +31,8 @@ TEST(ConcurrentMarking) {
   if (!i::FLAG_concurrent_marking) return;
   CcTest::InitializeVM();
   Heap* heap = CcTest::heap();
+  CcTest::CollectAllGarbage();
+  if (!heap->incremental_marking()->IsStopped()) return;
   ConcurrentMarking::MarkingWorklist shared, bailout;
   WeakObjects weak_objects;
   ConcurrentMarking* concurrent_marking =
@@ -45,6 +47,8 @@ TEST(ConcurrentMarkingReschedule) {
   if (!i::FLAG_concurrent_marking) return;
   CcTest::InitializeVM();
   Heap* heap = CcTest::heap();
+  CcTest::CollectAllGarbage();
+  if (!heap->incremental_marking()->IsStopped()) return;
   ConcurrentMarking::MarkingWorklist shared, bailout;
   WeakObjects weak_objects;
   ConcurrentMarking* concurrent_marking =