LibWeb/CSS: Introduce a LengthOrAuto type

`clip: rect()` in particular wants this for its parameters.

Libraries/LibWeb/CSS/Length.h

@@ -251,6 +251,48 @@ private:
     double m_value { 0 };
 };
 
+class LengthOrAuto {
+public:
+    LengthOrAuto(Length length)
+    {
+        if (length.is_auto())
+            m_length = {};
+        else
+            m_length = move(length);
+    }
+
+    static LengthOrAuto make_auto() { return LengthOrAuto { OptionalNone {} }; }
+
+    bool is_length() const { return m_length.has_value(); }
+    bool is_auto() const { return !m_length.has_value(); }
+
+    Length const& length() const { return m_length.value(); }
+
+    String to_string(SerializationMode mode = SerializationMode::Normal) const
+    {
+        if (is_auto())
+            return "auto"_string;
+        return m_length->to_string(mode);
+    }
+
+    CSSPixels to_px_or_zero(Layout::Node const& node) const
+    {
+        if (is_auto())
+            return 0;
+        return m_length->to_px(node);
+    }
+
+    bool operator==(LengthOrAuto const&) const = default;
+
+private:
+    explicit LengthOrAuto(Optional<Length> maybe_length)
+        : m_length(move(maybe_length))
+    {
+    }
+
+    Optional<Length> m_length;
+};
+
 }
 
 template<>
@@ -260,3 +302,11 @@ struct AK::Formatter<Web::CSS::Length> : Formatter<StringView> {
         return Formatter<StringView>::format(builder, length.to_string());
     }
 };
+
+template<>
+struct AK::Formatter<Web::CSS::LengthOrAuto> : Formatter<StringView> {
+    ErrorOr<void> format(FormatBuilder& builder, Web::CSS::LengthOrAuto const& length_or_auto)
+    {
+        return Formatter<StringView>::format(builder, length_or_auto.to_string());
+    }
+};

Libraries/LibWeb/Forward.h

@@ -310,6 +310,7 @@ class InvalidationSet;
 class KeywordStyleValue;
 class Length;
 class LengthBox;
+class LengthOrAuto;
 class LengthOrCalculated;
 class LengthPercentage;
 class LengthStyleValue;