Add UseCounter for rel=stylesheet link headers

PreloadHelper::LoadLinksFromHeader is currently the only time we
consider loading link headers, so in order to count how many of these
link headers are not rel=preload, but rel=stylesheet, this seems to be
the best place to perform a count.

Bug: 19237
Change-Id: I1c5cb5eb2200875a56714484f40e955b27d13746
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1710505
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679551}