Implement TracePathToObjectFrom helper for debugging.

src/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1267 matching lines @@
   inline bool IsInGCPostProcessing() { return gc_post_processing_depth_ > 0; }
 
 #ifdef DEBUG
   bool IsAllocationAllowed() { return allocation_allowed_; }
   inline bool allow_allocation(bool enable);
 
   bool disallow_allocation_failure() {
     return disallow_allocation_failure_;
   }
 
+  void TracePathToObjectFrom(Object* target, Object* root);
   void TracePathToObject(Object* target);
   void TracePathToGlobal();
 #endif
 
   // Callback function passed to Heap::Iterate etc.  Copies an object if
   // necessary, the object might be promoted to an old space.  The caller must
   // ensure the precondition that the object is (a) a heap object and (b) in
   // the heap's from space.
   static inline void ScavengePointer(HeapObject** p);
   static inline void ScavengeObject(HeapObject** p, HeapObject* object);
@@ skipping 1460 matching lines @@
   AssertNoAllocation no_alloc;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG || LIVE_OBJECT_LIST
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_H_