Port the Simple_TextureCubemap sample to the new sample framework. Added helper functions for generating cube maps and sphere geometry. BUG=angle:521 Change-Id: Ic1e8584d69793233ec6a97744b13097d55b9f287 Reviewed-on: https://chromium-review.googlesource.com/188726 Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>