This class is a wrapper around gl::Data, aka the metadata describing
a gl::Context. We can use it for unit-testing validation functions
by passing that through the validation methods instead of a Context,
and using mocks to expect certain errors.

BUG=angleproject:747
BUG=angleproject:597

Change-Id: I7bd85c7662860ba9a3a13406579a088d447e5631
Reviewed-on: https://chromium-review.googlesource.com/309637Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>