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>
1 file changed