This implements getIndexRange for index validation, without any
caching. Vulkan does support a version of robust access, but it would
require the robust context creation attribute. Also, it differs slight
from the OpenGL spec.

Also note that this implementation does not create the index buffer
with the correct usage bits, but seems to work and doesn't produce an
error in the validation layers. We should probably update them.

This CL also doesn't impement index support for immediate data,
offsets, or the unsigned short index type.

BUG=angleproject:2167

Change-Id: I580930f85e23034b483f3ece62eb1faf8024d624
Reviewed-on: https://chromium-review.googlesource.com/681874
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>