Make SkImage own its GrContext.

Change-Id: I86577fab5406ae9ad89d87fa971b0db6e0283cb4
Reviewed-on: https://skia-review.googlesource.com/130020
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
8 files changed