Design doc pointer: https://docs.google.com/document/d/1rEBpw5V-Nn1UIi8CIFa5ZZvwlR08SkY3CogvWE2UMFs/edit#heading=h.69bgponrf8jk

Once committed interstitials have been implemented for certificate errors, we'll need to modify ChromeContentBrowserClient::AlowCertificateError to trigger the new code path. To do so, for main-frame navigations, it should immediately cancel the request instead of calling into SSLErrorHandler to show an interstitial.