Added SkImage::MakeCrossContextFromEncoded

Added SkImage::MakeCrossContextFromEncoded

Designed for Flutter's threading architecture, with
an eye to being useful to other clients. Under the
hood, uses a new image generator class to lazily wrap
a texture for multiple GrContexts.

Bug: skia:
Change-Id: I6c37b12c8ab5bce94b91190e5f0beb91d31ae81b
Reviewed-on: https://skia-review.googlesource.com/14180
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>