[sksg] Fix inval tracking in Group::removeChild()

TBR=
Change-Id: If3536f8be9d379402cde18c0482108fcf5ae01bb
Reviewed-on: https://skia-review.googlesource.com/158822
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed