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.h

@@ -30,6 +30,7 @@
 #include <LibCore/ElapsedTimer.h>
 #include <LibWeb/Bindings/Wrappable.h>
 #include <LibWeb/DOM/EventTarget.h>
+#include <LibWeb/NavigationTiming/PerformanceTiming.h>
 
 namespace Web::HighResolutionTime {
 
@@ -46,6 +47,8 @@ public:
     double now() const { return m_timer.elapsed(); }
     double time_origin() const;
 
+    RefPtr<NavigationTiming::PerformanceTiming> timing() { return *m_timing; }
+
     virtual void ref_event_target() override;
     virtual void unref_event_target() override;
 
@@ -55,6 +58,8 @@ public:
 private:
     DOM::Window& m_window;
     Core::ElapsedTimer m_timer;
+
+    OwnPtr<NavigationTiming::PerformanceTiming> m_timing;
 };
 
 }