Add "lax mixed content checking" runtime flag.

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

 function reportResult(msg) {
-    if (self.opener)
+    if ("opener" in self)
         self.opener.postMessage(msg, "*");
-    else if (self.top)
-        self.top.postMessage(msg, "*");
     else
         postMessage(msg);
 }
 
 (function () {
     try {
         var ws = new WebSocket("ws://127.0.0.1:8880/echo");
     } catch (e) {
         reportResult("DONE");
         return;
     }
     reportResult("FAIL: No exception was thrown")
 })();