No support for raw vertex data pointers yet.

BUG=angle:880

Change-Id: Ifa8099b0f49028a1465edecde495ba725ac79598
Reviewed-on: https://chromium-review.googlesource.com/252801Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>