LibWeb: Implement performance.now()

This patch introduces the HighResolutionTime namespace which is home to
the Performance object (exposed via window.performance)

performance.now() is currently the only function, and it returns the
number of milliseconds since the window object was constructed. :^)

Libraries/LibWeb/DOM/Window.cpp

@@ -30,6 +30,7 @@
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOM/Timer.h>
 #include <LibWeb/DOM/Window.h>
+#include <LibWeb/HighResolutionTime/Performance.h>
 #include <LibWeb/InProcessWebView.h>
 #include <LibWeb/Page/Frame.h>
 
@@ -42,6 +43,7 @@ NonnullRefPtr<Window> Window::create_with_document(Document& document)
 
 Window::Window(Document& document)
     : m_document(document)
+    , m_performance(make<HighResolutionTime::Performance>(*this))
 {
 }