For testing, support file and memory streams of files

Update the JPEG gainmap test to use both.

Change-Id: I08e3cc183fa0946122eafe80e28395651ab12fec
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/632581
Reviewed-by: Christopher Cameron <ccameron@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed