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When waiting for a reply from a request, XCB (under some circumstances)
may only flush the request queue up to the request in question.  This
may cause unnecessary round trips.  To avoid this, add an explicit flush
where necessary.

R=sky

Bug: None
Change-Id: Ifaa1a43103517668b963d644d7ee628bbd1dc935
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533397
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826966}