rename doDrawSlug to onDrawSlug

Change-Id: Ic88b400f7943804481934ba2c9c54aeb8e04121b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/523656
Commit-Queue: Herb Derby <herb@google.com>
Auto-Submit: Herb Derby <herb@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
4 files changed