[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.
Merge pull request #3573 from Rob--W/crx-view-local-files

[CRX] Fixes to view local files again
Adds reference for the Opera extension

Addresses https://github.com/mozilla/pdf.js/issues/3600#issuecomment-23018555
Really call unregister for _pdfRedirectorFactory (fixing issue #3598)
Merge pull request #3601 from jmozmoz/unregister

Really call unregister for _pdfRedirectorFactory (fixing issue #3598)
Renaming of extensions/chrome folder to extensions/chromium, and small tweak to manifest.json description as per #3600
adjusted paths in make.js to reflect chromium instead of chrome
adjusted some more chrome references in make.js and README.js as per @yurydelendik's suggestion
Merge pull request #3602 from operasoftware/master

Make Chrome extension vendor-neutral
Fixes 'make web' after chromium directory remaning