Implement computeScratchKey virtual for GrAttachment.

Was missing this virtual so when we were registering attachments in the
cache we were never setting a scratch key on them.

Bug: skia:10727
Change-Id: Id505fcea17ed83b9f9f33739ded518adbcc2acb7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/326440
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed