Avoid manual object's body traversal in GC.

src/heap/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 #include <map>
 
@@ skipping 2338 matching lines @@
 
   ArrayBufferTracker* array_buffer_tracker_;
 
   // Classes in "heap" can be friends.
   friend class AlwaysAllocateScope;
   friend class GCCallbacksScope;
   friend class GCTracer;
   friend class HeapIterator;
   friend class IdleScavengeObserver;
   friend class IncrementalMarking;
+  friend class IteratePointersToFromSpaceVisitor;
   friend class MarkCompactCollector;
   friend class MarkCompactMarkingVisitor;
   friend class NewSpace;
   friend class ObjectStatsVisitor;
   friend class Page;
   friend class Scavenger;
   friend class StoreBuffer;
 
   // The allocator interface.
   friend class Factory;
@@ skipping 351 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_HEAP_H_