content/2019/playground-v3: add talk on gVisor-based Playgound "v3" Change-Id: Ib22fbb76a2bd6f99b31a07a570d0a4e169b533ae Reviewed-on: https://go-review.googlesource.com/c/talks/+/200080 Reviewed-by: Andrew Bonventre <andybons@golang.org>