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.h

@@ -42,6 +42,7 @@ public:
     ~Performance();
 
     double now() const { return m_timer.elapsed(); }
+    double time_origin() const;
 
     virtual void ref_event_target() override;
     virtual void unref_event_target() override;