This is a step required before adding an ANGLE extension that will
allow passing the Visual ID to use for the context at display creation
time: the configs will have to be compatible with it even if the visual
has non-standard sizes.

BUG=522149

Change-Id: I85e8ee3b9072934c665bddfa2b461d26f8903ff3
Reviewed-on: https://chromium-review.googlesource.com/311220Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>