LibWeb: Cache lowercased tag name for getElementsByTagName() iteration

Instead of calling to_lowercase() on two strings for every step while
iterating over the HTMLCollection returned by getElementsByTagName(),
we now cache the lowercased tag name beforehand and reuse it.

2.4x speed-up on WebKit/PerformanceTests/DOM/DOMDivWalk.html

Userland/Libraries/LibWeb/DOM/Attr.h

@@ -29,7 +29,7 @@ public:
     DeprecatedFlyString const& namespace_uri() const { return m_qualified_name.namespace_(); }
     DeprecatedFlyString const& prefix() const { return m_qualified_name.prefix(); }
     DeprecatedFlyString const& local_name() const { return m_qualified_name.local_name(); }
-    DeprecatedString const& name() const { return m_qualified_name.as_string(); }
+    DeprecatedFlyString const& name() const { return m_qualified_name.as_string(); }
 
     DeprecatedString const& value() const { return m_value; }
     void set_value(DeprecatedString value);

Userland/Libraries/LibWeb/DOM/Element.cpp

@@ -705,7 +705,7 @@ void Element::make_html_uppercased_qualified_name()
 {
     // This is allowed by the spec: "User agents could optimize qualified name and HTML-uppercased qualified name by storing them in internal slots."
     if (namespace_() == Namespace::HTML && document().document_type() == Document::Type::HTML)
-        m_html_uppercased_qualified_name = qualified_name().to_uppercase();
+        m_html_uppercased_qualified_name = DeprecatedString(qualified_name()).to_uppercase();
     else
         m_html_uppercased_qualified_name = qualified_name();
 }

Userland/Libraries/LibWeb/DOM/Element.h

@@ -71,7 +71,7 @@ class Element
 public:
     virtual ~Element() override;
 
-    DeprecatedString const& qualified_name() const { return m_qualified_name.as_string(); }
+    DeprecatedFlyString const& qualified_name() const { return m_qualified_name.as_string(); }
     DeprecatedString const& html_uppercased_qualified_name() const { return m_html_uppercased_qualified_name; }
     virtual DeprecatedFlyString node_name() const final { return html_uppercased_qualified_name(); }
     DeprecatedFlyString const& local_name() const { return m_qualified_name.local_name(); }

Userland/Libraries/LibWeb/DOM/ParentNode.cpp

@@ -133,10 +133,11 @@ JS::NonnullGCPtr<HTMLCollection> ParentNode::get_elements_by_tag_name(Deprecated
 
     // 2. Otherwise, if root’s node document is an HTML document, return a HTMLCollection rooted at root, whose filter matches the following descendant elements:
     if (root().document().document_type() == Document::Type::HTML) {
-        return HTMLCollection::create(*this, HTMLCollection::Scope::Descendants, [qualified_name](Element const& element) {
+        auto qualified_name_in_ascii_lowercase = qualified_name.to_lowercase();
+        return HTMLCollection::create(*this, HTMLCollection::Scope::Descendants, [qualified_name, qualified_name_in_ascii_lowercase](Element const& element) {
             // - Whose namespace is the HTML namespace and whose qualified name is qualifiedName, in ASCII lowercase.
             if (element.namespace_() == Namespace::HTML)
-                return element.qualified_name().to_lowercase() == qualified_name.to_lowercase();
+                return element.qualified_name() == qualified_name_in_ascii_lowercase;
 
             // - Whose namespace is not the HTML namespace and whose qualified name is qualifiedName.
             return element.qualified_name() == qualified_name;

Userland/Libraries/LibWeb/DOM/QualifiedName.h

@@ -19,7 +19,7 @@ public:
     DeprecatedFlyString const& prefix() const { return m_impl->prefix; }
     DeprecatedFlyString const& namespace_() const { return m_impl->namespace_; }
 
-    DeprecatedString const& as_string() const { return m_impl->as_string; }
+    DeprecatedFlyString const& as_string() const { return m_impl->as_string; }
 
     struct Impl : public RefCounted<Impl> {
         Impl(DeprecatedFlyString const& local_name, DeprecatedFlyString const& prefix, DeprecatedFlyString const& namespace_);
@@ -29,7 +29,7 @@ public:
         DeprecatedFlyString local_name;
         DeprecatedFlyString prefix;
         DeprecatedFlyString namespace_;
-        DeprecatedString as_string;
+        DeprecatedFlyString as_string;
     };
 
     void set_prefix(DeprecatedFlyString const& value);