Add a ValidationContext class helper. 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/309637 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>