LibWeb: Implement relative lengths in media queries :^)

As noted, this is not entirely right, since we are using the computed
font's metrics instead of the initial font's metrics, but we do not
have a good way to obtain the latter.

Userland/Libraries/LibWeb/CSS/MediaQuery.h

@@ -135,7 +135,7 @@ private:
     {
     }
 
-    static bool compare(MediaFeatureValue left, Comparison comparison, MediaFeatureValue right);
+    static bool compare(DOM::Window const& window, MediaFeatureValue left, Comparison comparison, MediaFeatureValue right);
 
     struct Range {
         MediaFeatureValue left_value;