Add skstd::string_view::substr

Change-Id: I06ba9dd9ed8af8555233ddfa10d3e0ec6babc2ea
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/444759
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
2 files changed