...in Runtime_StringToArray.

When a string is sliced from an externalized two-byte string that has
only one-byte chars, String::IsFlat() and
should not call ToOneByteVector() on it and instead we should use

String: :IsOneByteRepresentation() can both be true, while
FlatContent: :IsOneByte() returns false. In this case we
String: :Get() to get the individual characters.
Bug: chromium:1350270
Change-Id: I735408602072279f09b32e1997c97b2900942bdd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3813070Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Cr-Commit-Position: refs/heads/main@{#82268}