Convert WARN_UNUSED_RESULT to [[nodiscard]] in //ipc.

This is an automated conversion produced by the script and command in
the attached bug.

Bug: 1287045
Change-Id: I809b657b451040ab4e5db4da5117ae125e711a52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3387844


Auto-Submit: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#959305}