Simplify format_rowbyts calculation.

As per comment in change 114371.

Change-Id: I9724d5df205bb1430ebd3371677cce344372decc
Reviewed-on: https://skia-review.googlesource.com/115610
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed