Initialize PDFView.url = ''

The absence of this property caused the following error in PDFView.download:

   Cannot read property 'split' of undefined

This property was unset when the PDF was loaded through a typed array instead
of a URL. With the fix, the file name will default to "document.pdf", as
defined by getPDFFileNameFromURL in web/ui_utils.js
Merge pull request #5083 from Rob--W/pdfview-url-property

Initialize PDFView.url = ''