It's safe to read from a multiview framebuffer if it is layered and
has just one view. The native OVR_multiview spec supports this as
well.

BUG=angleproject:2062
TEST=angle_end2end_tests

Change-Id: I04e1364390574075f7e06e39a64e3bf05a539a05
Reviewed-on: https://chromium-review.googlesource.com/1156509
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>