encapsulate GrTextBlob::Key creation into Key::Make

Change-Id: Iabd75ca316e7f0ae28d2fc24d29d42db804ed8d4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/381636
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Chris Dalton <csmartdalton@google.com>
3 files changed