[skottie] Add support for external font loading

Web fonts are quite common in Lottie text assets.  Expand the
ResourceProvider interface to allow loading of external (web) font data.

Fonts are passed back to Skottie as opaque SkData, and instantiated
using SkFontMgr::makeFromData().

Change-Id: Ia4c994bd60e6a3bd47c90172c267ed5f897a38e4
Reviewed-on: https://skia-review.googlesource.com/155601
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Ben Wagner <bungeman@google.com>
3 files changed