For use_x11 && use_ozone build, use X11 backend by default.

For ozone only, continue using Ozone impl, which is basically
gbm.

Bug: chromium:1085700
Change-Id: Iac104e983135e566b0d42a58e00d3c83b430d6dc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2346276Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>