make GrTextBlob::subRunList() const

Change-Id: I02ffb7c09b1b1507700d618451a64bf89dd6953f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/374357
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
2 files changed