Firefox pdf viewer support for e10s (Bug 942707).

See https://bugzilla.mozilla.org/show_bug.cgi?id=942707
Hard-coding the preference names here seems unfortunate to me, since it unnecessarily increases the maintenance burden; and it's easy for this file and [web/default_preferences.js](https://github.com/mozilla/pdf.js/blob/master/web/default_preferences.js) to get out of sync.

I would instead suggest using the preprocessor to include `DEFAULT_PREFERENCES` in this file, and then compare against those in the code below; for reference see either [bootstrap.js#L54](https://github.com/mozilla/pdf.js/blob/master/extensions/firefox/bootstrap.js#L54) or [PdfJs.jsm#63](https://github.com/mozilla/pdf.js/blob/master/extensions/firefox/content/PdfJs.jsm#L63).