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

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 78166d00f32c96017f99c3b3e66db30b085c366a..6a407e4ce1665bc2093e7bed4a8e7e0b3617a877 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -1498,7 +1498,6 @@ MaybeObject* Heap::AllocatePartialMap(InstanceType instance_type,
 
   // Map::cast cannot be used due to uninitialized map field.
   reinterpret_cast<Map*>(result)->set_map(raw_unchecked_meta_map());
-  reinterpret_cast<Map*>(result)->set_heap(this);
   reinterpret_cast<Map*>(result)->set_instance_type(instance_type);
   reinterpret_cast<Map*>(result)->set_instance_size(instance_size);
   reinterpret_cast<Map*>(result)->set_visitor_id(
@@ -1520,7 +1519,6 @@ MaybeObject* Heap::AllocateMap(InstanceType instance_type, int instance_size) {
 
   Map* map = reinterpret_cast<Map*>(result);
   map->set_map(meta_map());
-  map->set_heap(this);
   map->set_instance_type(instance_type);
   map->set_visitor_id(
       StaticVisitorBase::GetVisitorId(instance_type, instance_size));