Introduce a flag for tracing retaining path in GC.

src/heap/objects-visiting.h

Index: src/heap/objects-visiting.h
diff --git a/src/heap/objects-visiting.h b/src/heap/objects-visiting.h
index a442867569606c1d4471a3ee2720a1fc89cf5b13..a6261739b60872eb37e8c247cfabee63ec3e4784 100644
--- a/src/heap/objects-visiting.h
+++ b/src/heap/objects-visiting.h
@@ -381,6 +381,12 @@ VisitorDispatchTable<typename StaticNewSpaceVisitor<StaticVisitor>::Callback>
     StaticNewSpaceVisitor<StaticVisitor>::table_;
 
 
+#ifdef TRACE_RETAINING_PATH
+void SetCurrentRetainer(HeapObject* obj);
+void ResetCurrentRetainer(HeapObject* obj);
+#endif
+
+
 // Base class for visitors used to transitively mark the entire heap.
 // IterateBody returns nothing.
 // Certain types of objects might not be handled by this base class and
@@ -401,7 +407,13 @@ class StaticMarkingVisitor : public StaticVisitorBase {
   static void Initialize();
 
   INLINE(static void IterateBody(Map* map, HeapObject* obj)) {
+#ifdef TRACE_RETAINING_PATH
+    SetCurrentRetainer(obj);
+#endif
     table_.GetVisitor(map)(map, obj);
+#ifdef TRACE_RETAINING_PATH
+    ResetCurrentRetainer(obj);
+#endif
   }
 
   INLINE(static void VisitPropertyCell(Map* map, HeapObject* object));