Switched SkSLSection over to use StringFragment

We were previously using String, but there's no reason to not just use
a view.

Change-Id: I49d973f22590d98a2894501d945cfd671220657d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/417002
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
4 files changed