LibWeb: Fix bug in checking if GridTrackPlacement is auto-positioned

This fixes something I thought I had already fixed everywhere, where
previously there wasn't the possibility to have an Auto
GridTrackPlacement and so the Auto "implementation" was simply checking
if the position of the track was 0.

Userland/Libraries/LibWeb/CSS/StyleValue.cpp

@@ -1389,7 +1389,7 @@ bool FrequencyStyleValue::equals(StyleValue const& other) const
 
 String GridTrackPlacementShorthandStyleValue::to_string() const
 {
-    if (m_end->grid_track_placement().position() == 0)
+    if (m_end->grid_track_placement().is_auto())
         return String::formatted("{}", m_start->grid_track_placement().to_string());
     return String::formatted("{} / {}", m_start->grid_track_placement().to_string(), m_end->grid_track_placement().to_string());
 }