Use an ES3 shader spec for ES3 contexts.

BUG=angle:823

Change-Id: I377b4483b3835660e7fd45d923bf7f560cf2e5f6
Reviewed-on: https://chromium-review.googlesource.com/227593
Tested-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>