https://chromium-review.googlesource.com/c/chromium/src/+/625337 makes it so that NavigationThrottle::WillFailRequest can return custom error page HTML which will be committed as the error page.

For Supervised User interstitials, we additionally need NavigationThrottle::WillStartRequest and WillRedirectRequest to be able to provide custom error page HTML when they cancel requests. These methods can already provide error page HTML in the ThrottleCheckResult, but it's not plumbed through to commit it in the error page.

To do so, we need to add an optional error_page_html argument to NavigationRequest::OnRequestFailedInternal, and pass error_page_html to CommitErrorPage(). OnStartChecksComplete() and OnRedirectChecksComplete() should pass the error page HTML from the ThrottleCheckResult into OnRequestFailedInternal.

We will also need to remove the DCHECKs in OnStartChecksComplete and OnRedirectChecksComplete that check that CANCEL uses a net error code of ERR_ABORTED. Throttles should be able to cancel requests with an error code of their choice, just as they can from WillFailRequest.