Add unit test for bitmap-backed SkImage cache key extraction

Change-Id: I679699159baf368deeddc20e2910f15a98e3e114
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/746479
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed