Add a sample that uses multiple windows. BUG=angleproject:521 Change-Id: I50858193518b4d07edcb2073caaa99ce37fcc4c3 Reviewed-on: https://chromium-review.googlesource.com/267000 Reviewed-by: Brandon Jones <bajones@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>