[CRX] Fixes to view local files again

pdfHandler-local.js references the isPdfDownloadable function from
pdfHandler.js, but the function didn't expect that the responseHeaders
property was absent. Added a check to prevent a runtime error when a
local file is displayed in a frame, and show local PDF files again.

Local files are rendered on the `chrome-extension:`-protocol. The previous
method of getting the PDF URL was incorrect, this has been fixed as well.