Re-land rev. 12849 and 12868 (Heavy cleanup of the external pointer API + related fix).

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index d5815f2ecffa12f71ad39ab5a89c21088d92bf4e..ab9ce1af127c07043a0909ec3908cb1b90a14696 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -134,6 +134,14 @@ bool Object::IsFixedArrayBase() {
 }
 
 
+// External objects are not extensible, so the map check is enough.
+bool Object::IsExternal() {
+  return Object::IsHeapObject() &&
+      HeapObject::cast(this)->map() ==
+      HeapObject::cast(this)->GetHeap()->external_map();
+}
+
+
 bool Object::IsInstanceOf(FunctionTemplateInfo* expected) {
   // There is a constraint on the object; check.
   if (!this->IsJSObject()) return false;