glDrawElements will now work with indices and/or vertex data in client memory,
as well as in a buffer object.
Enable corresponding tests.

BUG=angleproject:1683

Change-Id: Iefb9796a48b21ed6f9a837b08b0ad3218ff6dd6b
Reviewed-on: https://chromium-review.googlesource.com/915721
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>