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/Forward.h

@@ -142,6 +142,10 @@ namespace Web::HighResolutionTime {
 class Performance;
 }
 
+namespace Web::NavigationTiming {
+class PerformanceTiming;
+}
+
 namespace Web::SVG {
 class SVGElement;
 class SVGGeometryElement;
@@ -274,6 +278,7 @@ class ImageDataWrapper;
 class LocationObject;
 class MouseEventWrapper;
 class NodeWrapper;
+class PerformanceTimingWrapper;
 class PerformanceWrapper;
 class ScriptExecutionContext;
 class SubmitEventWrapper;