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

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 1ad9b0f6e46678783b46e0a981f1f15d58f188fc..aa541f72b0e576cf957f81ce15255804177b0c80 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2738,21 +2738,14 @@ Code* Code::GetCodeFromTargetAddress(Address address) {
 
 
 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);