Add helpers for checking which extensions are available instead of
discarding the extension strings after function loading.

BUG=angleproject:1144

Change-Id: Ie3d209cd5f6f116bbedaa618a59a4c2d7ddda4d6
Reviewed-on: https://chromium-review.googlesource.com/296360Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>