Add histogram enum for third party VPN

This is now plumbed up in shill; update histograms.xml to match.

BUG=486182
TEST=trybots

Review-Url: https://codereview.chromium.org/2534073002
Cr-Commit-Position: refs/heads/master@{#434895}

tools/metrics/histograms/histograms.xml

@@ -104629,6 +104629,7 @@ value.
 <enum name="VPNDriver" type="int">
   <int value="0" label="OpenVPN"/>
   <int value="1" label="L2TP/IPSec"/>
+  <int value="2" label="Third Party"/>
 </enum>
 
 <enum name="VPNRemoteAuthenticationType" type="int">