WebGL 2.0 explicitly specifies it to be an error when gl_FragData is
indexed with anything else than constant zero in spec section
'GLSL ES 1.00 Fragment Shader Output'.

This doesn't apply to WebGL 1.0 or GLES.
dEQP-GLES2.functional.shaders.fragdata* test that dynamic indexing of
gl_FragData is allowed.

TEST=angle_unittests
BUG=angleproject:1210

Change-Id: Ib401242e7867f5e7943456b059dd8e24dc404098
Reviewed-on: https://chromium-review.googlesource.com/312045Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>