LibWeb: Make the node mutation algorithms more spec compliant

The mutation algorithms now more closely follow the spec and
fixes some assertion failures in tests such as Acid3 and Dromaeo.

The main thing that is missing right now is passing exceptions to the
bindings layer. This is because of issue #6075. I spent a while trying
to work it out and got so frustrated I just left it as a FIXME. Besides
that, the algorithms bail at the appropriate points.

This also makes the adopting steps in the document more spec compliant
as it's needed by the insertion algorithm. While I was at it, I added
the adoptNode IDL binding.

This adds a bunch of ancestor/descendant checks to TreeNode as well.
I moved the "remove_all_children" function to Node as it needs to use
the full remove algorithm instead of simply removing it from
the child list.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -43,6 +43,7 @@
 #include <LibWeb/DOM/Event.h>
 #include <LibWeb/DOM/ExceptionOr.h>
 #include <LibWeb/DOM/Range.h>
+#include <LibWeb/DOM/ShadowRoot.h>
 #include <LibWeb/DOM/Text.h>
 #include <LibWeb/DOM/Window.h>
 #include <LibWeb/Dump.h>
@@ -124,7 +125,7 @@ void Document::removed_last_ref()
                 VERIFY(&node.document() == this);
                 VERIFY(!node.is_document());
                 if (node.parent())
-                    node.parent()->remove_child(node);
+                    node.remove();
             }
         }
 
@@ -282,9 +283,7 @@ void Document::set_title(const String& title)
         head_element->append_child(*title_element);
     }
 
-    while (RefPtr<Node> child = title_element->first_child())
-        title_element->remove_child(child.release_nonnull());
-
+    title_element->remove_all_children(true);
     title_element->append_child(adopt(*new Text(*this, title)));
 
     if (auto* page = this->page()) {
@@ -659,12 +658,52 @@ NonnullRefPtrVector<HTML::HTMLScriptElement> Document::take_scripts_to_execute_a
     return move(m_scripts_to_execute_as_soon_as_possible);
 }
 
-void Document::adopt_node(Node& subtree_root)
+// https://dom.spec.whatwg.org/#concept-node-adopt
+void Document::adopt_node(Node& node)
 {
-    subtree_root.for_each_in_inclusive_subtree([&](auto& node) {
-        node.set_document({}, *this);
-        return IterationDecision::Continue;
-    });
+    auto& old_document = node.document();
+    if (node.parent())
+        node.remove();
+
+    if (&old_document != this) {
+        // FIXME: This should be shadow-including.
+        node.for_each_in_inclusive_subtree([&](auto& inclusive_descendant) {
+            inclusive_descendant.set_document({}, *this);
+            // FIXME: If inclusiveDescendant is an element, then set the node document of each attribute in inclusiveDescendant’s attribute list to document.
+            return IterationDecision::Continue;
+        });
+
+        // FIXME: For each inclusiveDescendant in node’s shadow-including inclusive descendants that is custom,
+        //        enqueue a custom element callback reaction with inclusiveDescendant, callback name "adoptedCallback",
+        //        and an argument list containing oldDocument and document.
+
+        // FIXME: This should be shadow-including.
+        node.for_each_in_inclusive_subtree([&](auto& inclusive_descendant) {
+            inclusive_descendant.adopted_from(old_document);
+            return IterationDecision::Continue;
+        });
+    }
+}
+
+// https://dom.spec.whatwg.org/#dom-document-adoptnode
+NonnullRefPtr<Node> Document::adopt_node_binding(NonnullRefPtr<Node> node)
+{
+    if (is<Document>(*node)) {
+        dbgln("Document::adopt_node_binding: Cannot adopt a document into a document (FIXME: throw as NotSupportedError exception, see issue #6075");
+        return node;
+    }
+
+    if (is<ShadowRoot>(*node)) {
+        dbgln("Document::adopt_node_binding: Cannot adopt a shadow root into a document (FIXME: throw as HierarchyRequestError exception, see issue #6075");
+        return node;
+    }
+
+    if (is<DocumentFragment>(*node) && downcast<DocumentFragment>(*node).host())
+        return node;
+
+    adopt_node(*node);
+
+    return node;
 }
 
 const DocumentType* Document::doctype() const