Add timestamp to the page rendered event.
Add timestamp to the page rendered event.

This is needed to track rendering time in Firefox's talos performance
framework. See https://bugzilla.mozilla.org/show_bug.cgi?id=1565680

Shoobx/pdf.js,caremerge/pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,nawawi/pdf.js,showpad/mozilla-pdf.js,Snuffleupagus/pdf.js,showpad/mozilla-pdf.js,mozilla/pdf.js,Snuffleupagus/pdf.js,Shoobx/pdf.js,timvandermeij/pdf.js,macroplant/pdf.js,xavier114fch/pdf.js,macroplant/pdf.js,xavier114fch/pdf.js,timvandermeij/pdf.js,Shoobx/pdf.js,macroplant/pdf.js,caremerge/pdf.js,nawawi/pdf.js,timvandermeij/pdf.js,mainegreen/pdf.js,Snuffleupagus/pdf.js,mozilla/pdf.js,nawawi/pdf.js,mainegreen/pdf.js