LibWeb: Use CSS::Size for computed size and max-size values

This patch changes the *computed* representation of the following CSS
properties to use CSS::Size:

- width, min-width, max-width
- height, min-height, max-height

A few things had to change in order for things to keep working,
but I tried to keep the diff to a minimum.

The main trouble was that `min-width` and `max-width` can't actually be
`auto`, but they *can* be `none`. We previously treated `auto` as a
valid value (and it behaved mostly like `none`).

Userland/Libraries/LibWeb/Layout/LayoutState.cpp

@@ -184,7 +184,7 @@ void LayoutState::UsedValues::set_node(NodeWithStyleAndBoxModelMetrics& node, Us
 
     auto const& computed_values = node.computed_values();
 
-    auto is_definite_size = [&](CSS::LengthPercentage const& size, float& resolved_definite_size, bool width) {
+    auto is_definite_size = [&](CSS::Size const& size, float& resolved_definite_size, bool width) {
         // A size that can be determined without performing layout; that is,
         // a <length>,
         // a measure of text (without consideration of line-wrapping),
@@ -206,6 +206,8 @@ void LayoutState::UsedValues::set_node(NodeWithStyleAndBoxModelMetrics& node, Us
         }
 
         if (size.is_length()) {
+            if (size.length().is_calculated())
+                return false;
             resolved_definite_size = size.length().to_px(node);
             return true;
         }