This makes it possible to do a standalone gn build of the ozone/gbm back
end by using the following gn args.
  use_ozone = true
  ozone_platform_gbm = true

BUG=angleproject:1569

Change-Id: I70716b83943dae95df48b847ff8cca5e2c555c4a
Reviewed-on: https://chromium-review.googlesource.com/749867
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>