Move SkRefSet / pipe code to smart pointer.

This clarifies the ownership and simplifies the code.

Change-Id: I18ce981aaabfdd4749a344276bd19d62aa2c3210
Reviewed-on: https://skia-review.googlesource.com/140350
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
3 files changed