Added enums to allow users to request major and minor versions of the
underlying API and if a WARP device is used.

BUG=angle:490

Change-Id: I0bfb2ac8d327da28a47cc8e6346300e47ab9538c
Reviewed-on: https://chromium-review.googlesource.com/225081Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>