Migrate some callers to optional-based GetNormalizedHeader overload

This doesn't migrate every callsite; it just migrates the ones where
the optional-based overload is clearly more ergonomic.

Change-Id: I01feb24e5bde8f48ff57e5f963b0ea2f4eef40c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5935252
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Commit-Queue: Chris Fredrickson <cfredric@chromium.org>
Reviewed-by: Ryan Sultanem <rsult@google.com>
Cr-Commit-Position: refs/heads/main@{#1369953}