Switched SkSLSetting over to using StringFragment

Change-Id: Ic089c867f292e47f5f5835f5a90c61d42e50ba07
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/417197
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
4 files changed