LibWeb: Add a constant for the number of longhand properties

We use this in multiple places (and will in more places in the future)
so it's worth having as a constant from a clarity point of view

Libraries/LibWeb/CSS/CSSStyleProperties.cpp

@@ -128,7 +128,7 @@ size_t CSSStyleProperties::length() const
     if (is_computed()) {
         if (!owner_node().has_value())
             return 0;
-        return to_underlying(last_longhand_property_id) - to_underlying(first_longhand_property_id) + 1;
+        return number_of_longhand_properties;
     }
 
     return m_properties.size();

Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -1058,7 +1058,7 @@ void StyleComputer::collect_animation_into(DOM::Element& element, Optional<CSS::
     auto compute_keyframe_values = [refresh, &computed_properties, &element, &pseudo_element, this](auto const& keyframe_values) {
         HashMap<PropertyID, RefPtr<StyleValue const>> result;
         HashMap<PropertyID, PropertyID> longhands_set_by_property_id;
-        auto property_is_set_by_use_initial = MUST(Bitmap::create(to_underlying(last_longhand_property_id) - to_underlying(first_longhand_property_id) + 1, false));
+        auto property_is_set_by_use_initial = MUST(Bitmap::create(number_of_longhand_properties, false));
 
         auto property_is_logical_alias_including_shorthands = [&](PropertyID property_id) {
             if (property_is_shorthand(property_id))

Libraries/LibWeb/CSS/StylePropertyMapReadOnly.cpp

@@ -150,7 +150,6 @@ WebIDL::UnsignedLong StylePropertyMapReadOnly::size() const
             // value on this"
             // Ensure style is computed on the element before we try to read it.
             element.document().update_style();
-            auto longhands_count = to_underlying(last_longhand_property_id) - to_underlying(first_longhand_property_id) + 1;
 
             // Some custom properties set on the element might also be in the registered custom properties set, so we
             // want the size of the union of the two sets.
@@ -160,7 +159,7 @@ WebIDL::UnsignedLong StylePropertyMapReadOnly::size() const
             for (auto const& [key, _] : element.document().registered_custom_properties())
                 custom_properties.set(key);
 
-            return longhands_count + custom_properties.size();
+            return number_of_longhand_properties + custom_properties.size();
         },
         [](GC::Ref<CSSStyleDeclaration> const& declaration) { return declaration->length(); });
 }

Meta/Lagom/Tools/CodeGenerators/LibWeb/GenerateCSSPropertyID.cpp

@@ -300,6 +300,7 @@ constexpr PropertyID first_inherited_longhand_property_id = PropertyID::@first_i
 constexpr PropertyID last_inherited_longhand_property_id = PropertyID::@last_inherited_longhand_property_id@;
 constexpr PropertyID first_longhand_property_id = PropertyID::@first_longhand_property_id@;
 constexpr PropertyID last_longhand_property_id = PropertyID::@last_longhand_property_id@;
+constexpr size_t number_of_longhand_properties = to_underlying(last_longhand_property_id) - to_underlying(first_longhand_property_id) + 1;
 
 enum class Quirk {
     // https://quirks.spec.whatwg.org/#the-hashless-hex-color-quirk