switch away from std::function in public api

Bug: skia:
Change-Id: I181382dc1f9d8671b814134c1a787185745b90a8
Reviewed-on: https://skia-review.googlesource.com/25643
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Reed <reed@google.com>
3 files changed