Allow multiple closed observers on GrOpsTask.

Bug: skia:10188

Change-Id: Ibed96f10c07feddbbf1e78d014c8ea346c91e53c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/287002
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
3 files changed