Some Isolate usage cleanups in objects.{h,cc}.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index a5bac543c528a352df3ee737d9dfb9efd61304c2..e0ea06486f009db8c9f96c541fbf6b3bf85fdcd4 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -3776,7 +3776,8 @@ class Map: public HeapObject {
   inline int visitor_id();
   inline void set_visitor_id(int visitor_id);
 
-  // Returns the heap this map belongs to.
+  // Returns the isolate/heap this map belongs to.
+  inline Isolate* isolate();
   inline Heap* heap();
 
   typedef void (*TraverseCallback)(Map* map, void* data);