The {Vector} class does not use it any more. External uses should be
converted to {size_t} instead of {int}.
This CL removes the function from vector.h and updates all users to
either use {size_t}, or cast to {int} explicitly. In tests, no further
checks are needed if the string is a constant.

R=mstarzinger@chromium.org

Bug: v8:9183
Change-Id: I60f99302504c74d8a7c79b147ca01d8ba61b6879
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1587393Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61092}