Change image encode api to return sk_sp

Bug: skia:
Change-Id: I238289bc630be27795cb1384955dd6e887597c05
Reviewed-on: https://skia-review.googlesource.com/22208
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
13 files changed