Use SW backend as ref for AsyncReadPixels unit test.

As regular GPU and async read pixels share more it doesn't make sense
to test the async path using the regular path as the reference.

Bug: skia:9269

Change-Id: I8c32289947a7ae8b0f6e4e25ce8a588dfa5413f3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/244512
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed