Implement FenceNVGL. BUG=angleproject:888 Change-Id: Iea6993fe5459cf829f4bd23b0df5e223f22903f5 Reviewed-on: https://chromium-review.googlesource.com/264989 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>