Added helper functions for generating cube maps and sphere geometry.

BUG=angle:521

Change-Id: Ic1e8584d69793233ec6a97744b13097d55b9f287
Reviewed-on: https://chromium-review.googlesource.com/188726Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>