This is to make testing parse tasks, which currently only work with
external strings, easier.

BUG=v8:6093

Change-Id: Ia248ea9e9547604b4c81179e54d8ef564377e2a9
Reviewed-on: https://chromium-review.googlesource.com/480559
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44715}