LibWeb: Implement "destroy" operation for documents

This is basically the old "discard" updated for the world of navigables.

Co-authored-by: Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -42,6 +42,7 @@
 #include <LibWeb/HTML/CustomElements/CustomElementDefinition.h>
 #include <LibWeb/HTML/CustomElements/CustomElementReactionNames.h>
 #include <LibWeb/HTML/CustomElements/CustomElementRegistry.h>
+#include <LibWeb/HTML/DocumentState.h>
 #include <LibWeb/HTML/EventLoop/EventLoop.h>
 #include <LibWeb/HTML/EventNames.h>
 #include <LibWeb/HTML/HTMLAnchorElement.h>
@@ -2410,6 +2411,39 @@ void Document::discard()
     // FIXME: 9. For each workletGlobalScope in document's worklet global scopes, terminate workletGlobalScope.
 }
 
+// https://html.spec.whatwg.org/multipage/document-lifecycle.html#destroy-a-document
+void Document::destroy()
+{
+    // 1. Destroy the active documents of each of document's descendant navigables.
+    for (auto navigable : descendant_navigables()) {
+        if (auto document = navigable->active_document())
+            document->destroy();
+    }
+
+    // 2. Set document's salvageable state to false.
+    m_salvageable = false;
+
+    // FIXME: 3. Run any unloading document cleanup steps for document that are defined by this specification and other applicable specifications.
+
+    // 4. Abort document.
+    abort();
+
+    // 5. Remove any tasks whose document is document from any task queue (without running those tasks).
+    HTML::main_thread_event_loop().task_queue().remove_tasks_matching([this](auto& task) {
+        return task.document() == this;
+    });
+
+    // 6. Set document's browsing context to null.
+    m_browsing_context = nullptr;
+
+    // 7. Set document's node navigable's active session history entry's document state's document to null.
+    navigable()->active_session_history_entry()->document_state->set_document(nullptr);
+
+    // FIXME: 8. Remove document from the owner set of each WorkerGlobalScope object whose set contains document.
+
+    // FIXME: 9. For each workletGlobalScope in document's worklet global scopes, terminate workletGlobalScope.
+}
+
 // https://html.spec.whatwg.org/multipage/browsing-the-web.html#abort-a-document
 void Document::abort()
 {

Userland/Libraries/LibWeb/DOM/Document.h

@@ -448,6 +448,8 @@ public:
     Vector<JS::Handle<HTML::Navigable>> descendant_navigables();
     Vector<JS::Handle<HTML::Navigable>> inclusive_descendant_navigables();
 
+    void destroy();
+
     // https://html.spec.whatwg.org/multipage/window-object.html#discard-a-document
     void discard();