Prevent using gl_ViewID_OVR as an l-value 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/497408 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>