Bug 911444 part 4. Expose a print method on PDF viewer windows. r=bholley

The method is exposed only if the consumer has the same principal as the PDF
would have if it were not getting the PDF viewer treatment.

The method just calls the print() method in the PDF viewer window.

It's enough to expose this on nsOuterWindowProxy, not RemoteOuterWindowProxy,
because PDF documents end up in the process they would have been in based on
their pre-PDF-viewer principal, since we do process determination in the parent
process but only run the pdfjs stream converter in the content process, once we
have decided which one to use.

Differential Revision: https://phabricator.services.mozilla.com/D63711