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

src/objects-inl.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 2720 matching lines @@
   // GetCodeFromTargetAddress might be called when marking objects during mark
   // sweep. reinterpret_cast is therefore used instead of the more appropriate
   // Code::cast. Code::cast does not work when the object's map is
   // marked.
   Code* result = reinterpret_cast<Code*>(code);
   return result;
 }
 
 
 Heap* Map::heap() {
-  Heap* heap = reinterpret_cast<Heap*>(READ_INTPTR_FIELD(this, kHeapOffset));
+  // NOTE: address() helper is not used to save one instruction.
+  Heap* heap = Page::FromAddress(reinterpret_cast<Address>(this))->heap_;
   ASSERT(heap != NULL);
   ASSERT(heap->isolate() == Isolate::Current());
   return heap;
 }
 
 
-void Map::set_heap(Heap* heap) {
-  ASSERT(heap != NULL);
-  ASSERT(heap->isolate() == Isolate::Current());
-  // WRITE_FIELD does not invoke write barrier, but there is no need here.
-  WRITE_INTPTR_FIELD(this, kHeapOffset, reinterpret_cast<intptr_t>(heap));
-}
-
-
 Object* Code::GetObjectFromEntryAddress(Address location_of_address) {
   return HeapObject::
       FromAddress(Memory::Address_at(location_of_address) - Code::kHeaderSize);
 }
 
 
 Object* Map::prototype() {
   return READ_FIELD(this, kPrototypeOffset);
 }
 
@@ skipping 1254 matching lines @@
 #undef WRITE_INT_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_