This is one step towards removing the {StrLength} helper and using
{size_t} consistently instead.

R=mstarzinger@chromium.org

Bug: v8:8834
Change-Id: Ibcdfd579531a259d490c39a8e8c96d469a5a4aac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1578901
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60974}