Some shaders fail to compile with validation error messages that make no sense.

BUG=angle:648,chromium:372016

Change-Id: Ief0e28efa6bc7eaa483c95f45c00ce4f9f20e541
Reviewed-on: https://chromium-review.googlesource.com/199353Tested-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>