This will help port OSWindow to other platforms.

BUG=angleproject:892

Change-Id: If9884d310d80421e2fc09cffd03bcf01327b1eee
Reviewed-on: https://chromium-review.googlesource.com/283900Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>