We will need this functionality for index validation, so we likely
can assume an implemenation will need to support such a call. We
might redesign this requirement, but for now we can't call though
to BufferD3D on OSX.

BUG=angle:773

Change-Id: I16187c78ff62471fdf7221a11b9956560e617a14
Reviewed-on: https://chromium-review.googlesource.com/238450Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>