It's a shader input and as such should not be writable.

BUG=angleproject:1669
TEST=angle_unittests

Change-Id: I05cb5c63b7272dfa6e80cad57385da02504e4d8f
Reviewed-on: https://chromium-review.googlesource.com/497408Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>