LibWeb: Make document-level style invalidation fast

Add a flag to DOM::Document that means the whole document needs a style
update. This saves us the trouble of traversing the entire DOM to mark
all nodes as needing a style update.

Userland/Libraries/LibWeb/DOM/Document.h

@@ -334,6 +334,9 @@ public:
             callback(*node_iterator);
     }
 
+    bool needs_full_style_update() const { return m_needs_full_style_update; }
+    void set_needs_full_style_update(bool b) { m_needs_full_style_update = b; }
+
 private:
     explicit Document(const AK::URL&);
 
@@ -438,6 +441,8 @@ private:
 
     bool m_needs_layout { false };
 
+    bool m_needs_full_style_update { false };
+
     HashTable<NodeIterator*> m_node_iterators;
 };