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

Review URL: https://codereview.chromium.org/1431443002

Cr-Commit-Position: refs/heads/master@{#31649}

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. */                                                       \

src/heap/heap.cc

@@ -792,6 +792,8 @@ void Heap::FinalizeIncrementalMarking(const char* gc_reason) {
   }
 
   GCTracer::Scope gc_scope(tracer(), GCTracer::Scope::MC_INCREMENTAL_FINALIZE);
+  HistogramTimerScope incremental_marking_scope(
+      isolate()->counters()->gc_incremental_marking_finalize());
 
   {
     GCCallbacksScope scope(this);

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;