LibWeb: Cache the first <base href> (in tree order) in Document

When parsing relative URLs, we have to check the first <base href> in
tree order (if one is available). This was getting *very* costly on
large DOMs with many relative urls.

This patch avoids all that repeated traversal by letting Document cache
the first <base href> and invalidating the cache whenever a <base>
element is added/removed/edited in the DOM.

The browser was stuck doing this for a *very* long time when loading
the ECMA-262 spec, and this removes that problem entirely.

Userland/Libraries/LibWeb/HTML/HTMLBaseElement.h

@@ -22,6 +22,7 @@ public:
     AK::URL const& frozen_base_url() const { return m_frozen_base_url; }
 
     virtual void inserted() override;
+    virtual void removed_from(Node*) override;
     virtual void parse_attribute(FlyString const& name, String const& value) override;
 
 private: