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After [1], all X11 usage is done on a single thread, so no locking is
necessary, and using the atomic xcb_send_request_with_fds() is no
longer necessary; instead, we can send the FDs followed by the request.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2343314

Change-Id: Id47a07e283965a5e72bfa4cc3266b448e986400c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2399502
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805139}