ONC: Reject ServerCARef in IPsec if PSK is used.

Looking at IpsecManager::Initialize in platform/vpn-manager/ipsec_manager.cc, then a CA certificate for server verification is rejected if a PSK is set.

However, in ONC, the ServerCARef was silently ignored if PSK was used. This might unintentionally reduce security.

Note: ServerCARef from ONC maps to server_ca_file in  IpsecManager::Initialize.
PSK from ONC maps to psk_file in IpsecManager::Initialize.

BUG=276291
R=bartfab@chromium.org

Review URL: https://codereview.chromium.org/62173002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240865 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240865