Previously the code was hitting a weirdness of glXCreateConfigAttrib
where a compatilibility context was created even if the core profile
bit was set.

BUG=chromium:694877

Change-Id: I17164d620b39a26d73e34e1fc8de0ef66aef80f4
Reviewed-on: https://chromium-review.googlesource.com/446673Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>