DisplayGLX: XSync before setting the error handler This mirrors https://codereview.chromium.org/2206973002 BUG=628823 Change-Id: Ifd71d67df174cac3f90097c809fc91046699bed8 Reviewed-on: https://chromium-review.googlesource.com/366790 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>