The patch fixes the bug of having the OVR_multiview extension directive
being outputted by the ESSL translator whenever the
SH_INITIALIZE_BUILTINS_FOR_INSTANCED_MULTIVIEW option is enabled. The
directive should not be outputted because the extension is emulated
through that option.

BUG=angleproject:2062
TEST=angle_unittests

Change-Id: I95d0a651ace6db42d496de08e774ec7ceca4c197
Reviewed-on: https://chromium-review.googlesource.com/558981
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>