LibWeb: Define if identifier represent area or line during layout [GFC]

This fixes regression introduced in
c03e025a32 by assuming that it is
possible to determine whether identifier stands for line or area
during parsing.

Tests/LibWeb/Layout/input/grid/placement-using-named-tracks-3.html

@@ -1,6 +1,4 @@
 <style>
-    /* FIXME: This test is not rendered correctly */
-
     * {
         border: 1px solid black;
     }