Use std::ignore rather than ignore_result in chromeos/

Bug: 1285085
Change-Id: Ieac3d74ae7f9b76d03171456bb00b3d139286c5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3374325


Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/main@{#957166}