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/DOM/Document.h

@@ -99,6 +99,9 @@ public:
     Frame* frame() { return m_frame.ptr(); }
     const Frame* frame() const { return m_frame.ptr(); }
 
+    Page* page();
+    const Page* page() const;
+
     Color background_color(const Gfx::Palette&) const;
     RefPtr<Gfx::Bitmap> background_image() const;