Fix up incorrect capitalization of gl_ViewID_OVR in a few places.

Also add extension macro test to compiler unit tests.

BUG=angleproject:1669
TEST=angle_unittests

Change-Id: Ia7fdd747ad08355cdc149db9e2e7911c2e673af5
Reviewed-on: https://chromium-review.googlesource.com/425851Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>