add bounds generation code to SkGlyphRun

Change-Id: I29c269bce86cb50fea57d44d7760b60326aa56a8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/396436
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
6 files changed