Make GlyphVector::flatten const

Change-Id: I4039f209c1a4185443fdd7319d9b3e712afbf29f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/556609
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed