[sksg] Refactor inval registration

... to avoid having too many Node friends.

TBR=
Change-Id: I8f8ff570d94ea48017935066a3d51cd8265ec120
Reviewed-on: https://skia-review.googlesource.com/97980
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
10 files changed