Introduce gyp flag 'use_ozone' to select Ozone code.
Ozone is a display method used on Chrome OS which is based on
gbm, drm/kms, and surfaceless egl.

BUG=angleproject:1297

Change-Id: Ic1f66121d23d26bac56b09e3e6aedefec5a131df
Reviewed-on: https://chromium-review.googlesource.com/338442
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>