LibWeb: Stub out the PerformanceTiming object from Navigation Timing

Just have all the timing functions return 0 for now.

We can now run the Shynet JS on https://linus.dev/ although the XHR
is rejected by our same-origin policy.

Userland/Libraries/LibWeb/HighResolutionTime/Performance.idl

@@ -1,4 +1,6 @@
 interface Performance : EventTarget {
     double now();
     readonly attribute double timeOrigin;
+
+    readonly attribute PerformanceTiming timing;
 };