[json] Repair JSON.parse regression with non-sequential strings.

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index 80fca71b2d7c884ed3b3672a369bca692ae174dc..ea2be0bcdfaf3d45c9ca24794a2df3b02962c977 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -2251,6 +2251,7 @@ BUILTIN(JsonParse) {
   Handle<String> string;
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, string,
                                      Object::ToString(isolate, source));
+  string = String::Flatten(string);
   RETURN_RESULT_OR_FAILURE(
       isolate, string->IsSeqOneByteString()
                    ? JsonParser<true>::Parse(isolate, string, reviver)