Add "Write Debug Trace/Dump" buttons to SkSL slide in Viewer.

Click these buttons to trace execution of the current SkSL program.
This demonstrates end-to-end debug trace functionality in
SkRuntimeEffect.

Change-Id: I684099e337d1d275e444eb33dfa3a9e99343bb17
Bug: skia:12708
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/481336
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
2 files changed