Mark the pack and unpack buffer types as valid in ES3. dEQP resetState generates a GL error and exits when trying to reset the pixel buffers after a test. BUG=angleproject:1149 Change-Id: I133788c6c2f17074bf5bbf5cd17b63338352765f Reviewed-on: https://chromium-review.googlesource.com/297081 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>