generalize text geometry back pointer

This change allows types other than GrTextBlob to use the
geometry's lifetime to manage refs.

Change-Id: Iacb726cd4d097c87df146e119c476c014d4df99e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/471616
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed