Make streamsPrivate optional; remove manifest key

Use streamsPrivate API when available.
When the API is not available, the extension will still work on
on http/https/file URLs, but not for POST requests or FTP.

As of writing, the Chromium project has still not whitelisted
the PDF Viewer extension in the Chrome Web Store.
 (extension ID oemmndcbldboiebfnladdacbdfmadadm)
Request to whitelist PDF.js in Chromium:
 https://code.google.com/p/chromium/issues/detail?id=326949

Opera 19 has whitelisted the PDF Viewer extension from
 https://addons.opera.com/extensions/details/pdf-viewer/
 (extension ID encfpfilknmenlmjemepncnlbbjlabkc)
 (https://github.com/Rob--W/pdf.js/issues/1#issuecomment-32357302)

If you want to test the streamsPrivate feature in Chrome,
edit the build/extensions/manifest.json and add the "key" again
 (see this commit for the value of this "key" field).