[heap] Add a flag to disable incremental marking tasks

The new flag controls foreground incremental marking tasks and does not
affect background concurent marking tasks.

Change-Id: I46bda6b10c76b7a7f2ed2e123afe85bc04935624
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2135733Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66988}

src/flags/flag-definitions.h

@@ -889,6 +889,7 @@ DEFINE_BOOL(trace_mutator_utilization, false,
 DEFINE_BOOL(incremental_marking, true, "use incremental marking")
 DEFINE_BOOL(incremental_marking_wrappers, true,
             "use incremental marking for marking wrappers")
+DEFINE_BOOL(incremental_marking_task, true, "use tasks for incremental marking")
 DEFINE_BOOL(trace_unmapper, false, "Trace the unmapping")
 DEFINE_BOOL(parallel_scavenge, true, "parallel scavenge")
 DEFINE_BOOL(scavenge_task, true, "schedule scavenge tasks")

src/heap/incremental-marking-job.cc

@@ -47,7 +47,8 @@ void IncrementalMarkingJob::Start(Heap* heap) {
 }
 
 void IncrementalMarkingJob::ScheduleTask(Heap* heap, TaskType task_type) {
-  if (!IsTaskPending(task_type) && !heap->IsTearingDown()) {
+  if (!IsTaskPending(task_type) && !heap->IsTearingDown() &&
+      FLAG_incremental_marking_task) {
     v8::Isolate* isolate = reinterpret_cast<v8::Isolate*>(heap->isolate());
     SetTaskPending(task_type, true);
     auto taskrunner =