Add helper method to clone and register a child FP.

This will remove common boilerplate from our gen-code, and gives us a
place to put common child-cloning boilerplate.

Change-Id: I6101655af89d4c5844ec908b81ce4f6e5d59f834
Bug: skia:10217
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/296177
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
25 files changed