If a user downgrades from Chromium 35+ to 34, then the PDF Viewer
extension will not work any more because the extension assumes
that certain features were available based on the cached feature
detection results.
To resolve this problem, all feature detection scripts run again
if the browser was downgraded.
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.
