Disable -Wunused-but-set-variable in various third_party directories.

Bug: 1203071
Change-Id: I04ee9d41de53bc5b09ad552fff8e76ca4399d9ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3216996


Commit-Queue: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/main@{#930624}
NOKEYCHECK=True
GitOrigin-RevId: 3884c1f0647dccabe81e148e0184a595c2e030d9