Add a WindowTest sample to test OSWindow manually This will help port OSWindow to other platforms. BUG=angleproject:892 Change-Id: If9884d310d80421e2fc09cffd03bcf01327b1eee Reviewed-on: https://chromium-review.googlesource.com/283900 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>