LibWeb: Don't tell UI process about subframe title changes

This fixes an issue where loading an iframe would cause the current
browser tab title to get overwritten with an empty string.

The problem is that nested browsing contexts can be considered "top
level" during their initialization, but only one browsing context is
ever the Page::top_level_browsing_context(), so that's what we check.

Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp

@@ -532,7 +532,7 @@ void BrowsingContext::set_active_document(JS::NonnullGCPtr<DOM::Document> docume
     // AD-HOC:
     document->set_browsing_context(this);
 
-    if (m_page && is_top_level())
+    if (m_page && this == &m_page->top_level_browsing_context())
         m_page->client().page_did_change_title(document->title());
 
     if (previously_active_document && previously_active_document != document.ptr())