Vulkan: support indices in client memory. 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>