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.cpp

@@ -44,6 +44,12 @@ Performance::~Performance()
 {
 }
 
+double Performance::time_origin() const
+{
+    auto origin = m_timer.origin_time();
+    return (origin.tv_sec * 1000.0) + (origin.tv_usec / 1000.0);
+}
+
 void Performance::ref_event_target()
 {
     m_window.ref();

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;

Libraries/LibWeb/HighResolutionTime/Performance.idl

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