LibWeb: Make Frame point weakly to Page

This patch makes Page weakable and allows page-less frames to exist.

Page is single-owner, and Frame is multiple-owner, so it's not sound
for Frame to assume its containing Page will stick around for its own
entire lifetime.

Fixes #3976.

Libraries/LibWeb/HTML/HTMLFormElement.cpp

@@ -105,7 +105,8 @@ void HTMLFormElement::submit(RefPtr<HTMLInputElement> submitter)
         request.set_body(body);
     }
 
-    document().frame()->page().load(request);
+    if (auto* page = document().page())
+        page->load(request);
 }
 
 }