LibWeb: Add hook to HtmlView when a new document is set

Author: https://github.com/FalseHonesty
Commit: 51e79a2bbc
Pull-request: https://github.com/SerenityOS/serenity/pull/2349
Reviewed-by: https://github.com/Dexesttp
Reviewed-by: https://github.com/linusg

Libraries/LibWeb/HtmlView.cpp

@@ -89,6 +89,9 @@ void HtmlView::set_document(Document* new_document)
 
     main_frame().set_document(new_document);
 
+    if (on_set_document)
+        on_set_document(new_document);
+
     if (new_document) {
         new_document->on_layout_updated = [this] {
             layout_and_sync_size();