The patch adds validation for the ANGLE_multiview end-points
and propagates the calls to the unimplemented stubs in Context.

The patch also removes the
glFramebufferTextureMultiviewSideBySideRobustANGLE end-point
because the size of the viewport offsets buffer can trivially
be computed from the number of views.

BUG=angleproject:2062

Change-Id: I9a10bc00c19825e586d3df2750fabea4daf5ef8f
Reviewed-on: https://chromium-review.googlesource.com/573861
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>