Move warning suppression flag to default_warnings config

It seems like the was accidentally added to the treat_warnings_as_errors
config instead of the regular suppression config, so the warning was
turned back on for some local builds where that gn arg was disabled.
Move it to the normal place for warning suppressions.

R=lukasza@chromium.org

Bug: 427702057
Change-Id: I28c532071d627a9d98375e7b6e8806bdcf5fa4f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6681120
Reviewed-by: Hans Wennborg <hans@chromium.org>
Auto-Submit: Devon Loehr <dloehr@google.com>
Commit-Queue: Devon Loehr <dloehr@google.com>
Cr-Commit-Position: refs/heads/main@{#1479887}