Use size_t for span and container sizes in lookalikes

base::span will shortly require that size field is of
type size_t and is thus unsigned.

This CL changes the types used in lookalike URL detection
to use size_t so this check will pass.

This is part of a larger CL https://crrev.com/c/4211460.

BUG=1385166

Change-Id: Ia93692e151337d3a9841b623d7832ebd5ac0e4f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4218658
Commit-Queue: Will Harris <wfh@chromium.org>
Reviewed-by: Joe DeBlasio <jdeblasio@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1100548}