Currently just for image drawable, but going to use this for
references to other kinds of data in bindings, too.
Change-Id: Ic6673530013337bbaadd2d3f1c040626ec24ffb8
Bug: skia:9513
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/256776
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
