url: Use string_views for CanonicalizeHostSubstring() arguments

Unlike other Canonicalize*Host*() functions, CanonicalizeHostSubstring()
assumed the `Component` argument was always valid, and it doesn't return
offsets in `spec`, so we can just pass a string_view for a host part.

This CL has no behavior changes.

Bug: 451652367
Change-Id: I3765ec4f3621877bf259e6260243c821ad752608
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7043083
Commit-Queue: Kent Tamura <tkent@chromium.org>
Owners-Override: Hayato Ito <hayato@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1529947}