sk_tool::Registry: make an iterator.

Change-Id: Icf4e31b50bbd91b7ea330a1300f736d6dfd0a41c
Reviewed-on: https://skia-review.googlesource.com/144500
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
12 files changed