The WebGL spec proposal was changed so that only GL_OVR_multiview
extension name is supported, instead of having two variants
OVR_multiview and OVR_multiview2. We're only supporting the WebGL
version of the shader extension, so we drop compiler support for
GL_OVR_multiview2. Shader restrictions were also removed from the
WebGL spec, so no special validation for how ViewID_OVR gets used is
needed.

Tests that were testing for the shader restrictions are either removed
or changed from negative tests to positive tests.

BUG=angleproject:1669
TEST=angle_unittests

Change-Id: I83f92b879376d41b727b5aca419fd75fb6f53477
Reviewed-on: https://chromium-review.googlesource.com/654608Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>