[skottie] "hold" keyframe support

TBR=

Change-Id: I5388bc5a5d24b3bbe3962b2da646719e95efe858
Reviewed-on: https://skia-review.googlesource.com/99281
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed