LibWeb: Give FormattingState a reference to its root state

FormattingStates can have parents, in case we're performing nested
layouts to determine something's intrinsic size. In those cases, it will
soon be useful to find the outermost (root) state.

Userland/Libraries/LibWeb/Layout/FormattingState.h

@@ -15,10 +15,23 @@
 namespace Web::Layout {
 
 struct FormattingState {
-    FormattingState() { }
+    FormattingState()
+        : m_root(*this)
+    {
+    }
+
     explicit FormattingState(FormattingState const* parent)
         : m_parent(parent)
+        , m_root(find_root())
+    {
+    }
+
+    FormattingState const& find_root() const
     {
+        FormattingState const* root = this;
+        for (auto* state = m_parent; state; state = state->m_parent)
+            root = state;
+        return *root;
     }
 
     struct NodeState {
@@ -92,6 +105,7 @@ struct FormattingState {
     HashMap<NodeWithStyleAndBoxModelMetrics const*, IntrinsicSizes> mutable intrinsic_sizes;
 
     FormattingState const* m_parent { nullptr };
+    FormattingState const& m_root;
 };
 
 Gfx::FloatRect absolute_content_rect(Box const&, FormattingState const&);