Change the client auth callback to the 'cert_cb' callback which leaves the
callback in charge of installing the certificate, private key, etc. This lets
us attach a certificate chain.
BUG=393323
Review URL: https://codereview.chromium.org/441823002
Cr-Commit-Position: refs/heads/master@{#289834}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289834 0039d316-1c4b-4281-b951-d872f2087c98