make GrTextBlob::canReuse const

Change-Id: I27b18e63f9f1ff4ae6ada70a7cf781d6e29c9108
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/374356
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
2 files changed