LibWeb: Let style elements remember which StyleSheetList they live in

Instead of trying to locate the relevant StyleSheetList on style element
removal from the DOM, we now simply keep a pointer to the list instead.

This fixes an issue where using attachShadow() on an element that had
a declarative shadow DOM would cause any style elements present to use
the wrong StyleSheetList when removing themselves from the tree.

Userland/Libraries/LibWeb/HTML/HTMLStyleElement.cpp

@@ -46,7 +46,7 @@ void HTMLStyleElement::inserted()
 
 void HTMLStyleElement::removed_from(Node* old_parent)
 {
-    m_style_element_utils.update_a_style_block(*this, old_parent);
+    m_style_element_utils.update_a_style_block(*this);
     Base::removed_from(old_parent);
 }