[heap] Add histogram timers for incremental marking start and finalization event.

src/heap/incremental-marking.cc

Index: src/heap/incremental-marking.cc
diff --git a/src/heap/incremental-marking.cc b/src/heap/incremental-marking.cc
index 5d05ddde2ce3c9cef2fca9411f430237d902f9d2..6d759253dff79dfa6362d97af8547ce733de1b5b 100644
--- a/src/heap/incremental-marking.cc
+++ b/src/heap/incremental-marking.cc
@@ -556,6 +556,8 @@ void IncrementalMarking::Start(const char* reason) {
   DCHECK(heap_->gc_state() == Heap::NOT_IN_GC);
   DCHECK(!heap_->isolate()->serializer_enabled());
 
+  HistogramTimerScope incremental_marking_scope(
+      heap_->isolate()->counters()->gc_incremental_marking_start());
   ResetStepCounters();
 
   was_activated_ = true;