This fixes %StringIteratorPrototype%.next to not mixup
UTF16 and UTF32, and consistently use UTF32 for now.

Bug: chromium:805855
Change-Id: If58e2fe0d9bebd894e12abf8af82881c74388294
Reviewed-on: https://chromium-review.googlesource.com/888741
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50886}