Added SkSL DSL SampleChild function

Change-Id: Icd94540bbf0afcc57cfd93f3b576249e26e25724
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/369776
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
7 files changed