Store WGL extensions in FunctionsWGL. 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/296360 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>