Document using Rubber Stamper for test expectations CLs Also includes a small update about FLAKY expectations. Bug: angleproject:6740 Change-Id: I4df6534412e3720fb9f7bdeeadc555892016b7fb Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3308830 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Yuly Novikov <ynovikov@chromium.org>