Make heap.h usable without objects-inl.h header.

src/heap/objects-visiting.h

Index: src/heap/objects-visiting.h
diff --git a/src/heap/objects-visiting.h b/src/heap/objects-visiting.h
index 03dcf757f32b2ce5f616f678afc1b4d07b37cc2b..5b150cf199a561799a6f7b810d0242a4d07902e8 100644
--- a/src/heap/objects-visiting.h
+++ b/src/heap/objects-visiting.h
@@ -148,14 +148,12 @@ class VisitorDispatchTable {
     }
   }
 
+  inline Callback GetVisitor(Map* map);
+
   inline Callback GetVisitorById(StaticVisitorBase::VisitorId id) {
     return reinterpret_cast<Callback>(callbacks_[id]);
   }
 
-  inline Callback GetVisitor(Map* map) {
-    return reinterpret_cast<Callback>(callbacks_[map->visitor_id()]);
-  }
-
   void Register(StaticVisitorBase::VisitorId id, Callback callback) {
     DCHECK(id < StaticVisitorBase::kVisitorIdCount);  // id is unsigned.
     callbacks_[id] = reinterpret_cast<base::AtomicWord>(callback);