[canvaskit] Add test and extern for MakeImage

This was originally exposed for use with specs/web-img-decode and has
seen some actual use.

Change-Id: I6cc4fdf431b45cbbc21b91881c688c5545ca44a3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/257317
Reviewed-by: Kevin Lubick <kjlubick@google.com>
4 files changed