[CRX] Fall back to sender.tab.url if !tab.url

sender.url is relatively new (Chromium 29)
 ( https://chromiumcodereview.appspot.com/14301016 )
[CRX] Remove redundant location.origin check

The PDF Viewer will always be rendered directly from the extension,
so checking "origin == chrome-extension:/.." is not needed any more.
( since edd4bae903c36048b7d4d4bed393e4408d0198ba )
Register as File browser handler in Chrome OS
Merge pull request #4191 from Rob--W/crx-chrome-os

Register PDF Viewer as PDF handler in Chrome OS's file browser