Correctly stringify mixed encoding indirect strings.

src/json-stringifier.h

Index: src/json-stringifier.h
diff --git a/src/json-stringifier.h b/src/json-stringifier.h
index 155ae5ffa9c5a0ca13e27b75ccc31e718e7837c0..0d17b356abb0a1ac1a8447954db81f1386483083 100644
--- a/src/json-stringifier.h
+++ b/src/json-stringifier.h
@@ -835,14 +835,14 @@ Vector<const uc16> BasicJsonStringifier::GetCharVector(Handle<String> string) {
 void BasicJsonStringifier::SerializeString(Handle<String> object) {
   object = FlattenGetString(object);
   if (is_ascii_) {
-    if (object->IsOneByteRepresentation()) {
+    if (object->IsOneByteRepresentationUnderneath()) {
       SerializeString_<true, uint8_t>(object);
     } else {
       ChangeEncoding();
       SerializeString(object);
     }
   } else {
-    if (object->IsOneByteRepresentation()) {
+    if (object->IsOneByteRepresentationUnderneath()) {
       SerializeString_<false, uint8_t>(object);
     } else {
       SerializeString_<false, uc16>(object);