Eagerly follow transitions to existing maps while json parsing.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 250e009b24f12c24582900bb2a4bcd89b77ab349..38f1657fa6c767a27e6db846fc39d830a72033fc 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -2815,6 +2815,14 @@ MUST_USE_RESULT Handle<Object> JSProxy::CallTrap(const char* name,
 }
 
 
+void JSObject::AddFastPropertyUsingMap(Handle<JSObject> object,
+                                       Handle<Map> map) {
+  CALL_HEAP_FUNCTION_VOID(
+      object->GetIsolate(),
+      object->AddFastPropertyUsingMap(*map));
+}
+
+
 MaybeObject* JSObject::SetPropertyForResult(LookupResult* result,
                                             String* name_raw,
                                             Object* value_raw,