GrTextBlob: remove unused hash function; add key() api

Change-Id: I20e4f2168713cbd54c0428c60e38a736cf735df3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/381476
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed