Add utility to override PreloadingConfig using RAII.

The previous approach of setting PreloadingConfig via a Feature override
and then having to remember to call ParseConfig before and after a test
was too brittle (and PreloadingConfig is not exposed outside of
content/).

Bug: 1464473
Change-Id: I0aefc189c020565ea7f839af22f0828424f68fc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4706092
Reviewed-by: Sreeja Kamishetty <sreejakshetty@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Simon Pelchat <spelchat@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1175701}