[canvaskit] Support children shaders into runtime shaders

Change-Id: I88106babe35f6e5b3ee764e7fbaf82c7f43d136d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/272646
Reviewed-by: Nathaniel Nifong <nifong@google.com>
6 files changed