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: I5272651bd1af5806b6b42067d2d0f0abf0895af3
Reviewed-on: https://chromium-review.googlesource.com/337781Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>