We checked this at link time in the D3D back-end, but this
restriction applies to all shaders.

TEST=dEQP-GLES2.functional.shaders.fragdata.*
BUG=angleproject:995
BUG=478572

Change-Id: I99111cc6aa05b9352693f9c3b5bc70d56c9842d4
Reviewed-on: https://chromium-review.googlesource.com/269846Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>