[skottie] add proxy to PropertyObserver

Change-Id: I5fa957c1f6d9ece6de4459cc590dec66fb8e0324
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/698057
Commit-Queue: Jorge Betancourt <jmbetancourt@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
4 files changed