LibWeb: Make StyleProperties::length_box() default to auto values

Undefined length values can default to auto in all length boxes and
we'll get the values we need. This saves us from having to deal with
undefined lengths later on in layout.

At some point we should break the style application process into
a few more formal steps, but this at least simplifies some things.

Libraries/LibWeb/CSS/StyleProperties.cpp

@@ -79,10 +79,10 @@ Length StyleProperties::length_or_fallback(CSS::PropertyID id, const Length& fal
 LengthBox StyleProperties::length_box(CSS::PropertyID left_id, CSS::PropertyID top_id, CSS::PropertyID right_id, CSS::PropertyID bottom_id) const
 {
     LengthBox box;
-    box.left = length_or_fallback(left_id, {});
-    box.top = length_or_fallback(top_id, {});
-    box.right = length_or_fallback(right_id, {});
-    box.bottom = length_or_fallback(bottom_id, {});
+    box.left = length_or_fallback(left_id, CSS::Length::make_auto());
+    box.top = length_or_fallback(top_id, CSS::Length::make_auto());
+    box.right = length_or_fallback(right_id, CSS::Length::make_auto());
+    box.bottom = length_or_fallback(bottom_id, CSS::Length::make_auto());
     return box;
 }