ANGLE creates a child window on X11, but Expose events need to be
forwarded to the parent window. Caused an issue on Chrome Linux with a
non-compositing window manager.

Bug: 991577
Change-Id: Ia240d83403ed28e8e9a6e87164327d6d683dd9ca
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1824036Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>