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

src/heap/objects-visiting-inl.h

Index: src/heap/objects-visiting-inl.h
diff --git a/src/heap/objects-visiting-inl.h b/src/heap/objects-visiting-inl.h
index 4c3235e7b3c4082b55bc6bbb3212a77691e71ee0..21b770d8c5a174f71b2d7fb578a29187603510b7 100644
--- a/src/heap/objects-visiting-inl.h
+++ b/src/heap/objects-visiting-inl.h
@@ -12,6 +12,13 @@
 namespace v8 {
 namespace internal {
 
+
+template <typename Callback>
+Callback VisitorDispatchTable<Callback>::GetVisitor(Map* map) {
+  return reinterpret_cast<Callback>(callbacks_[map->visitor_id()]);
+}
+
+
 template <typename StaticVisitor>
 void StaticNewSpaceVisitor<StaticVisitor>::Initialize() {
   table_.Register(