LibWeb: Tear down old layout tree when new document becomes active

When a new document becomes the active document of a browsing context,
we now notify the old document, allowing it to tear down its layout
tree. In the future, there might be more cleanups we'd like to do here.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -2256,4 +2256,9 @@ void Document::unload(bool recursive_flag, Optional<DocumentUnloadTimingInfo> un
     m_unload_counter -= 1;
 }
 
+void Document::did_stop_being_active_document_in_browsing_context(Badge<HTML::BrowsingContext>)
+{
+    tear_down_layout_tree();
+}
+
 }

Userland/Libraries/LibWeb/DOM/Document.h

@@ -440,6 +440,8 @@ public:
     DocumentUnloadTimingInfo const& previous_document_unload_timing() const { return m_previous_document_unload_timing; }
     void set_previous_document_unload_timing(DocumentUnloadTimingInfo const& previous_document_unload_timing) { m_previous_document_unload_timing = previous_document_unload_timing; }
 
+    void did_stop_being_active_document_in_browsing_context(Badge<HTML::BrowsingContext>);
+
 protected:
     virtual void visit_edges(Cell::Visitor&) override;
 

Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp

@@ -295,6 +295,8 @@ bool BrowsingContext::is_focused_context() const
 // https://html.spec.whatwg.org/multipage/browsers.html#set-the-active-document
 void BrowsingContext::set_active_document(JS::NonnullGCPtr<DOM::Document> document)
 {
+    auto previously_active_document = active_document();
+
     // 1. Let window be document's relevant global object.
     auto& window = verify_cast<HTML::Window>(relevant_global_object(document));
 
@@ -315,6 +317,9 @@ void BrowsingContext::set_active_document(JS::NonnullGCPtr<DOM::Document> docume
 
     if (m_page && is_top_level())
         m_page->client().page_did_change_title(document->title());
+
+    if (previously_active_document && previously_active_document != document.ptr())
+        previously_active_document->did_stop_being_active_document_in_browsing_context({});
 }
 
 void BrowsingContext::set_viewport_rect(Gfx::IntRect const& rect)