Avoid direct call from SkPictureData -> Slug

Chromium (the only user of Slug) will need to call
Slug::AddDeserialProcs(&procs) in order to register
the deserialization logic properly (will not be done by
default).

Change-Id: I04ab1be0dfd9cc27cc20d0f315f6665ec68913cd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/755550
Reviewed-by: Brian Osman <brianosman@google.com>
9 files changed