Prior to bug 1659432 we used to use a single nsPrintJob for the duration of a print session (from print preview open until we'd finished sending the print to a printer). For each nsPrintJob::PrintPreview() call on a given nsPrintJob (to update for changed print settings) we would update (replace) just the nsPrintJob's nsPrintData, using nsPrintJob to hold onto the unchanging data.

Since bug 1659432 we now discard the nsPrintJob itself whenever nsDocumentViewer::Print/nsDocumentViewer::PrintPreview is called. Given that, having nsPrintData separate from nsPrintJob is now confusing and unnecessary. We should merge the two classes to simplify things.