Simplify collection of async cookie checks for OS registrations

We avoid construction of two otherwise unnecessary vectors, and also
lift loop-invariant operations outside the loop.

Change-Id: Ia55c78591171d9998aa6519bd22a7f9dda049cfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5384134
Reviewed-by: Anthony Garant <anthonygarant@chromium.org>
Reviewed-by: Nan Lin <linnan@chromium.org>
Commit-Queue: Andrew Paseltiner <apaseltiner@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1277806}