[heap] Do not schedule MemoryReducer tasks when heap->use_tasks() is false

Bug: chromium:810739
Change-Id: I7658e01394b03ade1acedb37b1e56004dc7660fb
Reviewed-on: https://chromium-review.googlesource.com/959004Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51882}

src/heap/memory-reducer.cc

@@ -202,6 +202,7 @@ MemoryReducer::State MemoryReducer::Step(const State& state,
 
 void MemoryReducer::ScheduleTimer(double time_ms, double delay_ms) {
   DCHECK_LT(0, delay_ms);
+  if (!heap()->use_tasks()) return;
   // Leave some room for precision error in task scheduler.
   const double kSlackMs = 100;
   v8::Isolate* isolate = reinterpret_cast<v8::Isolate*>(heap()->isolate());