Reland "Notify RTC when OpsTask is closed so it can drop ownership"

This reverts commit 777f239a65a9ab0147ef5b81cc3b4c042b3f00d5.

Assigns observer in GrRenderTargetContext constructor.

Change-Id: Ie1d4a425eb959f7b822b5fc4988b2bb6ca2ed6a5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/284733
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
5 files changed