MarkVisitor      6611 src/heap.cc    class PathTracer::MarkVisitor: public ObjectVisitor {
MarkVisitor      6667 src/heap.cc      MarkVisitor mark_visitor(this);
MarkVisitor      2711 src/heap.h       class MarkVisitor;
MarkVisitor      2714 src/heap.h       void MarkRecursively(Object** p, MarkVisitor* mark_visitor);