Refactoring: Make AuthSessionStorage::Peek result const

Bug: b/238606050
Change-Id: I6f9c8c1b7e9b9dd15808aaaa6fc106d7ecf3c64d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4753034


Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Denis Kuznetsov <antrim@chromium.org>
Auto-Submit: Denis Kuznetsov <antrim@chromium.org>
Reviewed-by: Elie Maamari <emaamari@google.com>
Cr-Commit-Position: refs/heads/main@{#1180380}