Now when Ctrl/Cmd+S or "Save Page As" is used, Firefox will send PDF.js a
message to trigger downloading. This allows PDF.js to generate a new PDF
if there is modified form data that needs to be saved or send back the
unmodified data. Once PDF.js has generated the blob, it will send messages
to the PdfjsParent to open the "Save As" dialog.
Adds two tests:
1) Saving a plain PDF without forms.
2) Saving a PDF with modified forms and verifies the new PDF has the form
data.
Differential Revision: https://phabricator.services.mozilla.com/D87675
686ff6882f