LibWeb: Defer invalidation sets processing until update_style()

This change converts `Node::invalidate_style()` (invalidation sets
overload) from eagerly doing tree traversal that marks elements affected
by invalidation set to instead adding "pending invalidation sets" into
`StyleInvalidator`, processing of which is deferred until the next
`update_style()`. By doing that we sometimes substantially reduce amount
of work done performing tree traversal that marks elements for style
recalculation.

Improves performance on Discord, were according to my measurements we
were previously spending 20% of time in style invalidation, but now it's
down to <1%.

Libraries/LibWeb/DOM/StyleInvalidator.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <LibGC/CellAllocator.h>
+#include <LibWeb/CSS/InvalidationSet.h>
 #include <LibWeb/Forward.h>
 
 namespace Web::DOM {
@@ -16,7 +17,23 @@ class StyleInvalidator : public GC::Cell {
     GC_DECLARE_ALLOCATOR(StyleInvalidator);
 
 public:
+    void invalidate(Node& node);
+    void add_pending_invalidation(GC::Ref<Node>, CSS::InvalidationSet&&, bool invalidate_elements_that_use_css_custom_properties);
+    bool has_pending_invalidations() const { return !m_pending_invalidations.is_empty(); }
+
+    virtual void visit_edges(Cell::Visitor& visitor) override;
+
+private:
     void perform_pending_style_invalidations(Node& node, bool invalidate_entire_subtree);
+
+    struct PendingInvalidation {
+        bool invalidate_elements_that_use_css_custom_properties { false };
+        CSS::InvalidationSet invalidation_set;
+    };
+    HashMap<GC::Ref<Node>, PendingInvalidation> m_pending_invalidations;
+
+    Vector<CSS::InvalidationSet> m_subtree_invalidation_sets;
+    bool m_invalidate_elements_that_use_css_custom_properties { false };
 };
 
 }