Instanced multiview is fully implemented and performs much better than
broadcasting draw calls on the CPU. There's no need to maintain
translating ViewID_OVR to a uniform.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: If3a8fa573f37b7926c052aecfcd4271f266751dc
Reviewed-on: https://chromium-review.googlesource.com/819417Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>