Make OSWindow destructor virtual.

BUG=angle:734

Change-Id: I6deb639abc26a314dd890189613e0a3a2e1be1d2
Reviewed-on: https://chromium-review.googlesource.com/214714
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
2 files changed