[Tracing] Use TracingCategoryObserver in gc statistics

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 20eb44d834cf480d54bdd48bfea7c9e3653f5c34..a46f0233f383a67512f7ab008bc6192052c7ecfc 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -658,6 +658,7 @@ StartupData V8::WarmUpSnapshotDataBlob(StartupData cold_snapshot_blob,
 
 void V8::SetFlagsFromString(const char* str, int length) {
   i::FlagList::SetFlagsFromString(str, length);
+  i::FlagList::EnforceFlagImplications();
 }
 
 
@@ -8087,7 +8088,7 @@ size_t Isolate::NumberOfTrackedHeapObjectTypes() {
 bool Isolate::GetHeapObjectStatisticsAtLastGC(
     HeapObjectStatistics* object_statistics, size_t type_index) {
   if (!object_statistics) return false;
-  if (!i::FLAG_track_gc_object_stats) return false;
+  if (V8_LIKELY(!i::FLAG_gc_stats)) return false;
 
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
   i::Heap* heap = isolate->heap();