LibWeb: Implement performance.timeOrigin

This is the origin timestamp of the same monotonic clock used for the
performance.now() timestamp.

I got a little confused while implementing this, since the numbers are
very low. That's because it uses the CLOCK_MONOTONIC system clock,
which we start counting from 0 at boot. :^)

Libraries/LibWeb/HighResolutionTime/Performance.idl

@@ -1,3 +1,4 @@
 interface Performance : EventTarget {
     double now();
+    readonly attribute double timeOrigin;
 }