Annotation storage modification state is not cleared when printed

[timvandermeij]

Steps to reproduce:

1. Go to https://mozilla.github.io/pdf.js/web/viewer.html.
2. Open https://github.com/mozilla/pdf.js/blob/master/test/pdfs/annotation-text-widget.pdf.
3. Edit a text field.
4. Print the document (to a file or paper).
5. Navigate away from the viewer.

Notice you'll get a warning about unsaved work. I think this is because we handled this for saving a document, but not for printing it. I would expect it to behave the same, so when a print is made, the annotation storage modification state should be cleared.