Fix gray png BRD tests

Fix gray png BRD tests

Before my recent CL, these were not run at all (because the decode
color type was gray and the bitmap color type was alpha).
https://skia-review.googlesource.com/c/6088/

Now that these are being run, we should mark the bitmap as
gray so they draw correctly.

BUG=skia:

Change-Id: I9bdfe97671a60da2bcbef55377e1c1b2bd326f8f
Reviewed-on: https://skia-review.googlesource.com/6160
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>