Add histograms for idle notification and incremental marking steps

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 16d7a8040bd4bdc1307363521e0d3f1128cc04ad..fa67dc148e49c97ab7825e9ebea84ee44f3793d1 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -4227,6 +4227,9 @@ void Heap::MakeHeapIterable() {
 
 
 void Heap::AdvanceIdleIncrementalMarking(intptr_t step_size) {
+  HistogramTimerScope idle_notification_scope(
+      isolate_->counters()->gc_incremental_marking());
+
   incremental_marking()->Step(step_size,
                               IncrementalMarking::NO_GC_VIA_STACK_GUARD);
 
@@ -4263,6 +4266,9 @@ bool Heap::IdleNotification(int hint) {
   intptr_t step_size =
       size_factor * IncrementalMarking::kAllocatedThreshold;
 
+  HistogramTimerScope idle_notification_scope(
+      isolate_->counters()->gc_idle_notification());
+
   if (contexts_disposed_ > 0) {
     contexts_disposed_ = 0;
     int mark_sweep_time = Min(TimeMarkSweepWouldTakeInMs(), 1000);