LibWeb: Move Layout::Box::is_out_of_flow() to Layout::Node

I want to use this function in inline layout, where we're not just
dealing with boxes.

Userland/Libraries/LibWeb/Layout/Box.cpp

@@ -29,29 +29,6 @@ Box::~Box()
 {
 }
 
-// https://www.w3.org/TR/css-display-3/#out-of-flow
-bool Box::is_out_of_flow(FormattingContext const& formatting_context) const
-{
-    // A box is out of flow if either:
-
-    // 1. It is floated (which requires that floating is not inhibited).
-    if (!formatting_context.inhibits_floating() && computed_values().float_() != CSS::Float::None)
-        return true;
-
-    // 2. It is "absolutely positioned".
-    switch (computed_values().position()) {
-    case CSS::Position::Absolute:
-    case CSS::Position::Fixed:
-        return true;
-    case CSS::Position::Static:
-    case CSS::Position::Relative:
-    case CSS::Position::Sticky:
-        break;
-    }
-
-    return false;
-}
-
 void Box::set_needs_display()
 {
     if (!is_inline()) {

Userland/Libraries/LibWeb/Layout/Box.h

@@ -21,8 +21,6 @@ class Box : public NodeWithStyleAndBoxModelMetrics {
 public:
     Painting::PaintableBox const* paint_box() const;
 
-    bool is_out_of_flow(FormattingContext const&) const;
-
     virtual void set_needs_display() override;
 
     bool is_body() const;

Userland/Libraries/LibWeb/Layout/Node.cpp

@@ -33,6 +33,22 @@ Node::~Node()
         m_dom_node->set_layout_node({}, nullptr);
 }
 
+// https://www.w3.org/TR/css-display-3/#out-of-flow
+bool Node::is_out_of_flow(FormattingContext const& formatting_context) const
+{
+    // A layout node is out of flow if either:
+
+    // 1. It is floated (which requires that floating is not inhibited).
+    if (!formatting_context.inhibits_floating() && computed_values().float_() != CSS::Float::None)
+        return true;
+
+    // 2. It is "absolutely positioned".
+    if (is_absolutely_positioned())
+        return true;
+
+    return false;
+}
+
 bool Node::can_contain_boxes_with_position_absolute() const
 {
     return computed_values().position() != CSS::Position::Static || is<InitialContainingBlock>(*this);

Userland/Libraries/LibWeb/Layout/Node.h

@@ -74,6 +74,8 @@ public:
 
     bool is_inline_block() const;
 
+    bool is_out_of_flow(FormattingContext const&) const;
+
     // These are used to optimize hot is<T> variants for some classes where dynamic_cast is too slow.
     virtual bool is_box() const { return false; }
     virtual bool is_block_container() const { return false; }