Skip checking certificate properties for L2TP/IPsec VPN using pre-shared key.

NetworkConnectionHandler::VerifyConfiguredAndConnect() did not
differentiate between the pre-shared key and certificate flow when
verifying the certificate properties of a L2TP/IPsec VPN connection
request. It always threw a 'configuration required' error and caused the
VPN configuration dialog to pop up even when all the credentials
information was available. This CL fixes this issue.

BUG=307665
TEST=Verified the following scenarios:
1. Add a 'L2TP/IPsec + pre-shared key' VPN with 'Save identity and
   password' unchecked. Connect to the VPN and then disconnect. 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/161083005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251193 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251193