UnmarkVisitor    6627 src/heap.cc    class PathTracer::UnmarkVisitor: public ObjectVisitor {
UnmarkVisitor    6670 src/heap.cc      UnmarkVisitor unmark_visitor(this);
UnmarkVisitor    2712 src/heap.h       class UnmarkVisitor;
UnmarkVisitor    2715 src/heap.h       void UnmarkRecursively(Object** p, UnmarkVisitor* unmark_visitor);