Bug 848251 - Gather telemetry about character encoding menu use on Metro. r=mbrubeck.

browser/metro/base/content/contenthandlers/Content.js

@@ -385,6 +385,7 @@ let Content = {
       }
 
       case "Browser:SetCharset": {
+        docShell.gatherCharsetMenuTelemetry();
         docShell.charset = json.charset;
 
         let webNav = docShell.QueryInterface(Ci.nsIWebNavigation);