This is a follow-up to  bug 642025 . The prerenderer will fail with Unsupported Scheme status when it see a special data url () used to navigate to an error page. It would be helpful to the Offline Pages Background Loader to know the difference between a possibly transient network error and a repeatable problem with rendering the page (so it can decide whether to retry it later or not).

Some ideas from a mail discussion:

1. From mmenke@: use newer WebContentsObserver with IsErrorPage() support. Just this might allow differentiation between error page and unsupported scheme redirected from original url.

2. From clamy@: using the NavigationHandle interface to get error code for navigation to populate final status.

3. From carlosk@: why not support the data url? Then open question as to what happens subsequently for error page prerendering.