LibWeb: Stop tracing CSS style values

Remove the visit_edges hook from CSS::StyleValue and stop asking CSS
properties, descriptors, computed values, and layout nodes to trace
through their style values.

Style values are refcounted data objects, so they should not be part of
the GC graph. Keeping this cleanup separate makes the later layout tree
ownership change smaller and easier to review.

Libraries/LibWeb/CSS/CSSDescriptors.cpp

@@ -256,14 +256,6 @@ WebIDL::ExceptionOr<void> CSSDescriptors::set_css_text(StringView value)
     return {};
 }
 
-void CSSDescriptors::visit_edges(Visitor& visitor)
-{
-    Base::visit_edges(visitor);
-    for (auto& descriptor : m_descriptors) {
-        descriptor.value->visit_edges(visitor);
-    }
-}
-
 RefPtr<StyleValue const> CSSDescriptors::descriptor(DescriptorNameAndID const& descriptor_name_and_id) const
 {
     auto match = m_descriptors.first_matching([descriptor_name_and_id](Descriptor const& descriptor) {

Libraries/LibWeb/CSS/CSSDescriptors.h

@@ -39,8 +39,6 @@ protected:
 private:
     bool set_a_css_declaration(DescriptorNameAndID const&, NonnullRefPtr<StyleValue const>, Important);
 
-    virtual void visit_edges(Visitor&) override;
-
     AtRuleID m_at_rule_id;
     Vector<Descriptor> m_descriptors;
 };

Libraries/LibWeb/CSS/CSSStyleProperties.cpp

@@ -120,14 +120,6 @@ void CSSStyleProperties::initialize(JS::Realm& realm)
     Base::initialize(realm);
 }
 
-void CSSStyleProperties::visit_edges(Visitor& visitor)
-{
-    Base::visit_edges(visitor);
-    for (auto& property : m_properties) {
-        property.value->visit_edges(visitor);
-    }
-}
-
 size_t CSSStyleProperties::external_memory_size() const
 {
     auto size = Base::external_memory_size();

Libraries/LibWeb/CSS/CSSStyleProperties.h

@@ -74,7 +74,6 @@ private:
     CSSStyleProperties(JS::Realm&, Computed, Readonly, Vector<StyleProperty> properties, OrderedHashMap<FlyString, StyleProperty> custom_properties, Optional<DOM::AbstractElement>);
     static Vector<StyleProperty> convert_declarations_to_specified_order(Vector<StyleProperty>&);
 
-    virtual void visit_edges(Cell::Visitor&) override;
     virtual size_t external_memory_size() const override;
 
     RefPtr<StyleValue const> style_value_for_computed_property(Layout::NodeWithStyle const&, PropertyID) const;

Libraries/LibWeb/CSS/ComputedValues.h

@@ -487,15 +487,6 @@ struct ContentData {
     Vector<Variant<String, NonnullRefPtr<ImageStyleValue>>> data;
     Vector<ValueComparingRefPtr<CounterStyle const>> counter_style_dependencies;
     Optional<String> alt_text {};
-
-    void visit_edges(GC::Cell::Visitor& visitor) const
-    {
-        for (auto const& item : data) {
-            if (auto* ptr = item.get_pointer<NonnullRefPtr<ImageStyleValue>>()) {
-                (*ptr)->visit_edges(visitor);
-            }
-        }
-    }
 };
 
 struct CounterData {
@@ -546,11 +537,6 @@ public:
     ComputedValues() = default;
     ~ComputedValues() = default;
 
-    void visit_edges(GC::Cell::Visitor& visitor)
-    {
-        m_noninherited.visit_edges(visitor);
-    }
-
     AspectRatio aspect_ratio() const { return m_noninherited.aspect_ratio; }
     Float float_() const { return m_noninherited.float_; }
     Length border_spacing_horizontal() const { return m_inherited.border_spacing_horizontal; }
@@ -977,26 +963,6 @@ protected:
         Vector<CounterData, 0> counter_set;
         WillChange will_change { InitialValues::will_change() };
         Resize resize { InitialValues::resize() };
-
-        void visit_edges(GC::Cell::Visitor& visitor)
-        {
-            for (auto& layer : background_layers)
-                layer.background_image->visit_edges(visitor);
-            for (auto& layer : mask_layers)
-                layer.background_image->visit_edges(visitor);
-            if (mask_image)
-                mask_image->visit_edges(visitor);
-            for (auto const& transform : transformations)
-                transform->visit_edges(visitor);
-            if (rotate)
-                rotate->visit_edges(visitor);
-            if (translate)
-                translate->visit_edges(visitor);
-            if (scale)
-                scale->visit_edges(visitor);
-            if (content.has_value())
-                content->visit_edges(visitor);
-        }
     };
 
     NonInheritedValues m_noninherited;

Libraries/LibWeb/CSS/StyleValues/StyleValue.h

@@ -173,7 +173,6 @@ public:
     virtual StyleValueVector subdivide_into_iterations(PropertyNameAndID const&) const;
 
     virtual void set_style_sheet(GC::Ptr<CSSStyleSheet>) { }
-    virtual void visit_edges(JS::Cell::Visitor&) const { }
 
     virtual bool equals(StyleValue const& other) const = 0;
 

Libraries/LibWeb/Layout/Node.cpp

@@ -700,13 +700,6 @@ void NodeWithStyle::rebuild_image_observers()
 void NodeWithStyle::visit_edges(Visitor& visitor)
 {
     Base::visit_edges(visitor);
-    for (auto const& layer : computed_values().background_layers())
-        layer.background_image->visit_edges(visitor);
-
-    if (m_list_style_image)
-        m_list_style_image->visit_edges(visitor);
-
-    m_computed_values->visit_edges(visitor);
 }
 
 void NodeWithStyle::apply_style(CSS::ComputedProperties const& computed_style)