[Isolates] Get the current heap from page header instead of Map field.

src/objects.h

 // Copyright 2010 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 3539 matching lines @@
   // Dispatched behavior.
 #ifdef DEBUG
   void MapPrint();
   void MapVerify();
   void SharedMapVerify();
 #endif
 
   inline int visitor_id();
   inline void set_visitor_id(int visitor_id);
 
-  // Meta map has a heap pointer for fast access to Heap and Isolate.
+  // Returns the heap this map belongs to.
   inline Heap* heap();
-  inline void set_heap(Heap* heap);
 
   typedef void (*TraverseCallback)(Map* map, void* data);
 
   void TraverseTransitionTree(TraverseCallback callback, void* data);
 
   static const int kMaxPreAllocatedPropertyFields = 255;
 
   // Layout description.
-  static const int kHeapOffset = HeapObject::kHeaderSize;
-  static const int kInstanceSizesOffset = kHeapOffset + kPointerSize;
+  static const int kInstanceSizesOffset = HeapObject::kHeaderSize;
   static const int kInstanceAttributesOffset = kInstanceSizesOffset + kIntSize;
   static const int kPrototypeOffset = kInstanceAttributesOffset + kIntSize;
   static const int kConstructorOffset = kPrototypeOffset + kPointerSize;
   static const int kInstanceDescriptorsOffset =
       kConstructorOffset + kPointerSize;
   static const int kCodeCacheOffset = kInstanceDescriptorsOffset + kPointerSize;
   static const int kPadStart = kCodeCacheOffset + kPointerSize;
   static const int kSize = MAP_POINTER_ALIGN(kPadStart);
 
   // Layout of pointer fields. Heap iteration code relies on them
@@ skipping 2548 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_