Check configuration for L2TP/IPsec+certificate VPN network with UIData.

In the LT2P/IPsec + user certificate VPN flow, if the UIData does not
contain any certificate properties, it is possible that the certificate
properties are still configured by shill (e.g. the properties were
previously configured and saved in the shill profile). However,
client_cert::IsCertificateConfigured() did not take that into account,
which caused NetworkConnectionHandler::VerifyConfiguredAndConnect() to
always throw a 'configuration required' error and the VPN configuration
dialog to pop up even when all the credentials information was
available. Also, VPNRequiresCredentials didn't check the from the
Provider.PassphraseRequired property to see if shill expects a user
passphrase for the VPN connection. This CL fixes these issues.

BUG=307665
TEST=Verified the following scenarios:
1. Add a 'L2TP/IPsec + user certificate' VPN with 'Save identity and
   password' unchecked. Connect to the VPN once and then reboot the system.
   Reconnect to the VPN and verify that it prompts for credentials.
2. Repeat 1 but with 'Save identity and password' checked and verify
   that it reconnects without prompting for credentials.

R=pneubeck@chromium.org, stevenjb@chromium.org

Review URL: https://codereview.chromium.org/166063003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251410 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251410