LibWeb: Move perform_pending_style_invalidations() in StyleInvalidator

This change introduces StyleInvalidator as a preparation for upcoming
change that will make `perform_pending_style_invalidations()` take care
of pending invalidation sets.

Libraries/LibWeb/DOM/StyleInvalidator.cpp

@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) 2025, Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/DOM/Element.h>
+#include <LibWeb/DOM/Node.h>
+#include <LibWeb/DOM/ShadowRoot.h>
+#include <LibWeb/DOM/StyleInvalidator.h>
+
+namespace Web::DOM {
+
+GC_DEFINE_ALLOCATOR(StyleInvalidator);
+
+// This function makes a full pass over the entire DOM and converts "entire subtree needs style update"
+// into "needs style update" for each inclusive descendant where it's found.
+void StyleInvalidator::perform_pending_style_invalidations(Node& node, bool invalidate_entire_subtree)
+{
+    invalidate_entire_subtree |= node.entire_subtree_needs_style_update();
+
+    if (invalidate_entire_subtree) {
+        node.set_needs_style_update_internal(true);
+        if (node.has_child_nodes())
+            node.set_child_needs_style_update(true);
+    }
+
+    for (auto* child = node.first_child(); child; child = child->next_sibling()) {
+        perform_pending_style_invalidations(*child, invalidate_entire_subtree);
+    }
+
+    if (node.is_element()) {
+        auto& element = static_cast<Element&>(node);
+        if (auto shadow_root = element.shadow_root()) {
+            perform_pending_style_invalidations(*shadow_root, invalidate_entire_subtree);
+            if (invalidate_entire_subtree)
+                node.set_child_needs_style_update(true);
+        }
+    }
+
+    node.set_entire_subtree_needs_style_update(false);
+}
+
+}