[skottie] Ref-counted ResourceProvider

Update the ResourceProvider interface to inherit from SkRefCnt, to
clarify sharing/ownership semantics in the Skottie Builder API.  Now it
follows the same pattern as SkFontMgr.

Change-Id: I7ff8ad39023d9ecfe609e0180b5aabf776672d48
Reviewed-on: https://skia-review.googlesource.com/148991
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
4 files changed