Set disableCreateObjectURL for PDF.js

We can't use blob URLs or we'll get a renderer crash:
https://bugreports.qt.io/browse/QTBUG-70420

Thankfully, we can tell PDF.js to use data: URLs instead.

qutebrowser/browser/pdfjs.py

@@ -69,6 +69,7 @@ def _generate_pdfjs_script(url):
     """
     return (
         'document.addEventListener("DOMContentLoaded", function() {{\n'
+        '  PDFJS.disableCreateObjectURL = true;\n'
         '  PDFJS.verbosity = PDFJS.VERBOSITY_LEVELS.info;\n'
         '  (window.PDFView || window.PDFViewerApplication).open("{url}");\n'
         '}});\n'