There are three changes:
1. Adds a print progress listener to tabs.saveAsPDF() in ext-tabs.js so that the
'saved' or 'replaced' status is not returned until the PDF file has been saved.
2. Adds four more "edge" properties to the pageSettings object to allow positioning
of the page headers and footers.
3. Adds automated tests for tabs.saveAsPDF() in browser_ext_tabs_saveAsPDF.js that
cover all returned statuses: saved, replaced, canceled, not_saved, not_replaced.
MozReview-Commit-ID: iljvT8wp11