[Tracing] Use TracingCategoryObserver in gc statistics

src/heap/heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/heap/heap.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/ast/context-slot-cache.h"
 #include "src/base/bits.h"
@@ skipping 5452 matching lines @@
   for (int i = 0; i < static_cast<int>(v8::Isolate::kUseCounterFeatureCount);
        i++) {
     deferred_counters_[i] = 0;
   }
 
   tracer_ = new GCTracer(this);
   scavenge_collector_ = new Scavenger(this);
   mark_compact_collector_ = new MarkCompactCollector(this);
   gc_idle_time_handler_ = new GCIdleTimeHandler();
   memory_reducer_ = new MemoryReducer(this);
-  if (FLAG_track_gc_object_stats) {
+  if (V8_UNLIKELY(FLAG_gc_stats)) {
     live_object_stats_ = new ObjectStats(this);
     dead_object_stats_ = new ObjectStats(this);
   }
   scavenge_job_ = new ScavengeJob();
 
   LOG(isolate_, IntPtrTEvent("heap-capacity", Capacity()));
   LOG(isolate_, IntPtrTEvent("heap-available", Available()));
 
   store_buffer()->SetUp();
 
@@ skipping 996 matching lines @@
 }
 
 
 // static
 int Heap::GetStaticVisitorIdForMap(Map* map) {
   return StaticVisitorBase::GetVisitorId(map);
 }
 
 }  // namespace internal
 }  // namespace v8