Verify that both UTF-8 decoders (incremental and non-incremental one) match the
expectations.

Also cleanup / harden the UTF-8 handling code, as suggested in
https://chromium-review.googlesource.com/c/v8/v8/+/671020/ .


BUG=chromium:765608

Change-Id: I6344d62ca15b75ac8e333421c94c4aa35ab8190d
Reviewed-on: https://chromium-review.googlesource.com/681217
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48229}