A previous CL (https://crrev.com/c/1926769) changed hashing to always
treat the input as signed values. This causes problems, since the hash
of a one-byte string differs the hash of the identical two-byte string.
Hence this CL switches to treating all values as unsigned in hashing.

The bug cannot easily be reproduced in v8 alone, since we would need to
create an internalized two-byte string, which contains one-byte data.
Blink manages to create such a string via external strings.

R=jkummerow@chromium.org

Bug: chromium:1025184, chromium:1027131
Change-Id: Id41aa0e463691c02099a08c6e9d837a079c872df
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1930615Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65113}