Switch to using the public API similar to SkTextBlob for
serialize/Deserialize. Add deserializeSlug to SkStrikeClient.
Bug: chromium:1278340
Change-Id: I91b93487859c662e3bfdfba49ba4758f13529cd9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/511601
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>