LibWeb: Clear all active timers when document is destroyed

This change implements a step from the document's destroy procedure in
the specification, saying that all active timers should be cleared.

By doing this, we also fix the leaking of a document in case where we
have navigated away from a page that has scheduled timers that haven't
yet been triggered.

Userland/Libraries/LibWeb/HTML/Timer.cpp

@@ -34,6 +34,7 @@ void Timer::visit_edges(Cell::Visitor& visitor)
 
 Timer::~Timer()
 {
+    VERIFY(!m_timer->is_active());
 }
 
 void Timer::start()
@@ -41,4 +42,9 @@ void Timer::start()
     m_timer->start();
 }
 
+void Timer::stop()
+{
+    m_timer->stop();
+}
+
 }