[heap] Add trace events for catching RCS in tasks

Performance sheriffs: This CL adds better accounting and can thus cause
regressions.

Also-By: cbruni@chromium.org
Bug: 
Change-Id: I38cdb702af201033e180741af40c4c5915553798
Reviewed-on: https://chromium-review.googlesource.com/623867Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47476}

src/heap/incremental-marking-job.cc

@@ -42,6 +42,7 @@ void IncrementalMarkingJob::Task::Step(Heap* heap) {
 
 void IncrementalMarkingJob::Task::RunInternal() {
   VMState<GC> state(isolate());
+  TRACE_EVENT_CALL_STATS_SCOPED(isolate(), "v8.gc", "V8.Task");
   RuntimeCallTimerScope runtime_timer(
       isolate(), &RuntimeCallStats::GC_Custom_IncrementalMarkingJob);
 

src/heap/scavenge-job.cc

@@ -9,6 +9,7 @@
 #include "src/heap/heap.h"
 #include "src/isolate.h"
 #include "src/v8.h"
+#include "src/vm-state-inl.h"
 
 namespace v8 {
 namespace internal {
@@ -17,6 +18,8 @@ namespace internal {
 const double ScavengeJob::kMaxAllocationLimitAsFractionOfNewSpace = 0.8;
 
 void ScavengeJob::IdleTask::RunInternal(double deadline_in_seconds) {
+  VMState<GC> state(isolate());
+  TRACE_EVENT_CALL_STATS_SCOPED(isolate(), "v8.gc", "V8.Task");
   Heap* heap = isolate()->heap();
   double deadline_in_ms =
       deadline_in_seconds *