Makes it a bit easier to work with. In prep for adding offscreen
configs.

Bug: angleproject:3117
Change-Id: Ie2497574b2687592b7b3df2f7b933a19e83b6d16
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1650784
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>