Introduce a flag for tracing retaining path in GC.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index aa9358fa799e166d4c7b0da241fdc30dde999924..779ce3a8943d15c0b3210e36bd4469e72cf29b5f 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -7,6 +7,11 @@
 
 #include <cmath>
 
+#ifdef TRACE_RETAINING_PATH
+#include <map>
+#include <string>
+#endif
+
 #include "src/allocation.h"
 #include "src/assert-scope.h"
 #include "src/counters.h"
@@ -1444,6 +1449,12 @@ class Heap {
 
   bool deserialization_complete() const { return deserialization_complete_; }
 
+#ifdef TRACE_RETAINING_PATH
+  void SetCurrentRetainer(HeapObject* obj);
+  void SetCurrentRootRetainer(std::string root);
+  void AddRetainedObject(HeapObject* obj);
+#endif
+
  protected:
   // Methods made available to tests.
 
@@ -2124,6 +2135,15 @@ class Heap {
 
   bool deserialization_complete_;
 
+#ifdef TRACE_RETAINING_PATH
+  void ClearRetainingInfo();
+  void PrintRetainingPath(HeapObject* obj, const char* comment);
+  std::map<HeapObject*, HeapObject*> retainer_;
+  std::map<HeapObject*, std::string> root_retainer_;
+  HeapObject* current_retainer_;
+  std::string current_root_retainer_;
+#endif
+
   friend class AlwaysAllocateScope;
   friend class Deserializer;
   friend class Factory;