Use OSWindow with angle_tests for Window management. BUG=angle:730 Change-Id: I409fd4f4e00eb0d8d964b0ac199fa6f675a36df8 Reviewed-on: https://chromium-review.googlesource.com/213295 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Brandon Jones <bajones@chromium.org>