There were two separate bugs here. First, a signed/unsigned mismatch
where we took the result of PositiveNumberToUint32 and treated it as a
signed int. Second, AdvanceStringIndex did not handle large input
values correctly.

Both are fixed by using uint64_t consistently.

Bug: chromium:799813, v8:7258
Change-Id: If2819f87986d0ca732bc24df290f6dc7614083e8
Reviewed-on: https://chromium-review.googlesource.com/854272
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50432}