This was already the case for 1-byte strings. This prevents crashes when
attempting to externalize such strings.

Bug: chromium:842078, v8:7464
Change-Id: I3092a6748edaf77b2689f7b6f6b949929998e508
Reviewed-on: https://chromium-review.googlesource.com/1054290Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53124}