Improved FTP support for Chromium extension
Use redirectUrl at onHeadersReceived if available.

http://crbug.com/280464 has been resolved, so we can now use redirectUrl at
onHeadersReceived.
For backwards-compatibility, the code for the original method has not been
removed, and a feature detection script was added that detects whether the
desired feature is available.
Merge pull request #4545 from Rob--W/crx-api-enhancements

Use new Chromium APIs for FTP and redirects