Use an ES3 shader spec for ES3 contexts.
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 <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Tested-by: Zhenyao Mo <655bdf1d95ff956cd68886dd9e86ee5e987c9dbb@chromium.org>
Reviewed-by: Zhenyao Mo <655bdf1d95ff956cd68886dd9e86ee5e987c9dbb@chromium.org>
Reviewed-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@chromium.org>

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