split out serial functions for SkFont into separate impl

... in prep for future CL that makes serialiation more complicated
https://skia-review.googlesource.com/c/skia/+/252927

Change-Id: Ica3cb803c6cd47295b4f12574c9965e47dc57ac3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/252659
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Reed <reed@google.com>
3 files changed