LibWeb: Cache the result of Selector::specificity()

This function was showing up as taking 30% of all runtime during a
profile of Browser. This change effectively eliminates it completely.

Userland/Libraries/LibWeb/CSS/Selector.cpp

@@ -20,6 +20,9 @@ Selector::~Selector()
 
 u32 Selector::specificity() const
 {
+    if (m_specificity.has_value())
+        return *m_specificity;
+
     unsigned ids = 0;
     unsigned tag_names = 0;
     unsigned classes = 0;
@@ -42,7 +45,9 @@ u32 Selector::specificity() const
         }
     }
 
-    return ids * 0x10000 + classes * 0x100 + tag_names;
+    m_specificity = ids * 0x10000 + classes * 0x100 + tag_names;
+
+    return *m_specificity;
 }
 
 // https://www.w3.org/TR/cssom/#serialize-a-simple-selector

Userland/Libraries/LibWeb/CSS/Selector.h

@@ -138,6 +138,7 @@ private:
     explicit Selector(Vector<CompoundSelector>&&);
 
     Vector<CompoundSelector> m_compound_selectors;
+    mutable Optional<u32> m_specificity;
 };
 
 constexpr StringView pseudo_element_name(Selector::SimpleSelector::PseudoElement);