Add a vertex buffer example for GLES1 and 2.

Bug: angleproject:5751
Change-Id: If039451ff85dfffd8915497e9aaaab6e4ff71181
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2859827
Commit-Queue: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>