LibWeb: Calculate calc() lengths in to_px()

to_px() has the layout node available, so let's use it.

Userland/Libraries/LibWeb/CSS/Length.cpp

@@ -93,6 +93,9 @@ float Length::relative_length_to_px(Gfx::IntRect const& viewport_rect, Gfx::Font
 
 float Length::to_px(Layout::Node const& layout_node) const
 {
+    if (is_calculated())
+        return m_calculated_style->resolve_length(layout_node)->to_px(layout_node);
+
     if (!layout_node.document().browsing_context())
         return 0;
     auto viewport_rect = layout_node.document().browsing_context()->viewport_rect();

Userland/Libraries/LibWeb/CSS/Length.h

@@ -86,6 +86,8 @@ public:
             return 0;
         if (is_relative())
             return relative_length_to_px(viewport_rect, font_metrics, root_font_size);
+        if (is_calculated())
+            VERIFY_NOT_REACHED(); // We can't resolve a calculated length from here. :^(
         return absolute_length_to_px();
     }