Add "lax mixed content checking" runtime flag.

LayoutTests/http/tests/security/mixedContent/laxChecking/websocket/resources/expect-successful-construction.js

 function reportResult(msg) {
     if ("opener" in self)
         self.opener.postMessage(msg, "*");
     else
         postMessage(msg);
 }
 
 new Promise(function (resolve, reject) {
     var ws = new WebSocket("ws://127.0.0.1:8880/echo");
     ws.onopen = function () {
         resolve();
     };
     ws.onmessage = function () {
         reject("Unexpected message event");
     };
     ws.onerror = function () {
         reject("Unexpected error event");
     };
     ws.onclose = function () {
         reject("Unexpected close event before open event");
     };
 }).then(
     function () {
         reportResult("DONE");
     },
     function (reason) {
         reportResult("FAIL: " + reason);
     }
 );