Feature detection after downgrading the browser.

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.
Merge pull request #5090 from Rob--W/crx-feature-detect-downgrade

Re-run feature detection after downgrading the Chromium browser.