Profiler: Allow scrolling through full range of profile timeline

We were not taking the width of the process headers into account when
computing the scrollable content size of the timeline.

Fix this by passing the header width to AbstractScrollableWidget's
set_size_occupied_by_fixed_elements().

Userland/DevTools/Profiler/TimelineContainer.cpp

@@ -51,6 +51,7 @@ void TimelineContainer::update_widget_sizes()
         m_header_container->do_layout();
         auto preferred_size = m_header_container->layout()->preferred_size();
         m_header_container->resize(preferred_size);
+        set_size_occupied_by_fixed_elements({ preferred_size.width(), 0 });
     }
 }