Make incremental marking tracing format consistent with GC tracing.

src/heap/gc-tracer.h

Index: src/heap/gc-tracer.h
diff --git a/src/heap/gc-tracer.h b/src/heap/gc-tracer.h
index a11823e984f63ba1f25c3ddcca0c607e81571b50..cc36148156450bf1b4ea1860764481f778ad4f8a 100644
--- a/src/heap/gc-tracer.h
+++ b/src/heap/gc-tracer.h
@@ -381,6 +381,8 @@ class GCTracer {
   // Discard all recorded survival events.
   void ResetSurvivalEvents();
 
+  void NotifyIncrementalMarkingStart();
+
   V8_INLINE void AddScopeSample(Scope::ScopeId scope, double duration) {
     DCHECK(scope < Scope::NUMBER_OF_SCOPES);
     if (scope >= Scope::FIRST_INCREMENTAL_SCOPE &&
@@ -476,6 +478,8 @@ class GCTracer {
   // all sweeping operations performed on the main thread.
   double cumulative_sweeping_duration_;
 
+  double incremental_marking_start_time_;
+
   // Timestamp and allocation counter at the last sampled allocation event.
   double allocation_time_ms_;
   size_t new_space_allocation_counter_bytes_;