Called on the UI thread when a client certificate is being requested for authentication. Return false (0) to use the default behavior and automatically select the first certificate available. Return true (1) and call cef_select_client_certificate_callback_t::Select either in this function or at a later time to select a certificate. Do not call Select or call it with NULL to continue without using any certificate. |isProxy| indicates whether the host is an HTTPS proxy or the origin server. |host| and |port| contains the hostname and port of the SSL server. |certificates| is the list of certificates to choose from; this list has already been pruned by Chromium so that it only contains certificates from issuers that the server trusts.