[Tracing] Use TracingCategoryObserver in gc statistics

src/api.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/api.h"
 
 #include <string.h>  // For memcpy, strlen.
 #ifdef V8_USE_ADDRESS_SANITIZER
 #include <sanitizer/asan_interface.h>
 #endif  // V8_USE_ADDRESS_SANITIZER
@@ skipping 640 matching lines @@
     i::PrintF("Warming up snapshot took %0.3f ms\n",
               timer.Elapsed().InMillisecondsF());
   }
   timer.Stop();
   return result;
 }
 
 
 void V8::SetFlagsFromString(const char* str, int length) {
   i::FlagList::SetFlagsFromString(str, length);
+  i::FlagList::EnforceFlagImplications();
 }
 
 
 void V8::SetFlagsFromCommandLine(int* argc, char** argv, bool remove_flags) {
   i::FlagList::SetFlagsFromCommandLine(argc, argv, remove_flags);
 }
 
 
 RegisteredExtension* RegisteredExtension::first_extension_ = NULL;
 
@@ skipping 7409 matching lines @@
 size_t Isolate::NumberOfTrackedHeapObjectTypes() {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
   i::Heap* heap = isolate->heap();
   return heap->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();
   if (type_index >= heap->NumberOfTrackedHeapObjectTypes()) return false;
 
   const char* object_type;
   const char* object_sub_type;
   size_t object_count = heap->ObjectCountAtLastGC(type_index);
   size_t object_size = heap->ObjectSizeAtLastGC(type_index);
   if (!heap->GetObjectTypeName(type_index, &object_type, &object_sub_type)) {
@@ skipping 1669 matching lines @@
   Address callback_address =
       reinterpret_cast<Address>(reinterpret_cast<intptr_t>(callback));
   VMState<EXTERNAL> state(isolate);
   ExternalCallbackScope call_scope(isolate, callback_address);
   callback(info);
 }
 
 
 }  // namespace internal
 }  // namespace v8