Add unit test for pinned SkImages

Change-Id: I2e14353bc865b5994cc90ad643a6a58604955957
Reviewed-on: https://skia-review.googlesource.com/53500
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed