Force incremental marking steps in incremental-marking-job only if

the memory reducer is enabled.

BUG=

Review URL: https://codereview.chromium.org/1419393002

Cr-Commit-Position: refs/heads/master@{#31554}

src/heap/incremental-marking-job.cc

@@ -53,7 +53,7 @@ void IncrementalMarkingJob::ScheduleIdleTask(Heap* heap) {
 
 
 void IncrementalMarkingJob::ScheduleDelayedTask(Heap* heap) {
-  if (!delayed_task_pending_) {
+  if (!delayed_task_pending_ && FLAG_memory_reducer) {
     v8::Isolate* isolate = reinterpret_cast<v8::Isolate*>(heap->isolate());
     delayed_task_pending_ = true;
     made_progress_since_last_delayed_task_ = false;