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

src/counters.h

Index: src/counters.h
diff --git a/src/counters.h b/src/counters.h
index 9dff765dca0d18c3de01a663c98b3a142d95dc2b..0319f5b5ad4c9042c9d119e98f06d075147e9573 100644
--- a/src/counters.h
+++ b/src/counters.h
@@ -496,6 +496,10 @@ double AggregatedMemoryHistogram<Histogram>::Aggregate(double current_ms,
      MILLISECOND) /* GC context cleanup time */                               \
   HT(gc_idle_notification, V8.GCIdleNotification, 10000, MILLISECOND)         \
   HT(gc_incremental_marking, V8.GCIncrementalMarking, 10000, MILLISECOND)     \
+  HT(gc_incremental_marking_start, V8.GCIncrementalMarkingStart, 10000,       \
+     MILLISECOND)                                                             \
+  HT(gc_incremental_marking_finalize, V8.GCIncrementalMarkingFinalize, 10000, \
+     MILLISECOND)                                                             \
   HT(gc_low_memory_notification, V8.GCLowMemoryNotification, 10000,           \
      MILLISECOND)                                                             \
   /* Parsing timers. */                                                       \