Don't intrusively mark in the heap snapshot generator

src/profiler/heap-snapshot-generator.h

Index: src/profiler/heap-snapshot-generator.h
diff --git a/src/profiler/heap-snapshot-generator.h b/src/profiler/heap-snapshot-generator.h
index 3d6693b0d950701af6ce578ce17b739b477ac7dd..865dd56ad173e8bfe63cb912b8991164ccc16842 100644
--- a/src/profiler/heap-snapshot-generator.h
+++ b/src/profiler/heap-snapshot-generator.h
@@ -351,6 +351,8 @@ class V8HeapExplorer : public HeapEntriesAllocator {
   typedef bool (V8HeapExplorer::*ExtractReferencesMethod)(int entry,
                                                           HeapObject* object);
 
+  void MarkVisitedField(HeapObject* obj, int offset);
+
   HeapEntry* AddEntry(HeapObject* object);
   HeapEntry* AddEntry(HeapObject* object,
                       HeapEntry::Type type,
@@ -465,6 +467,8 @@ class V8HeapExplorer : public HeapEntriesAllocator {
   HeapObjectsSet weak_containers_;
   v8::HeapProfiler::ObjectNameResolver* global_object_name_resolver_;
 
+  std::vector<bool> marks_;
+
   friend class IndexedReferencesExtractor;
   friend class RootsReferencesExtractor;