Protect against deleting a current XFB buffer. Bug: chromium:1295411 Change-Id: I097f272c38e444e0af71aa55c0dc508a07aa0bd3 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3498262 Reviewed-by: Amirali Abdolrashidi <abdolrashidi@google.com> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>