LibWeb: Use coordinate instead of WeakPtr for box->fragment connection

Using WeakPtr to remember which LineBoxFragment owns which Box was
imposing some annoying constraints on the layout code. Importantly, it
was forcing us to heap-allocate fragments, which makes it much harder to
clone a FormattingState.

This patch replaces the WeakPtr with a coordinate system instead.
Fragments are referred to by their line box index + fragment index
within the line box.

Userland/Libraries/LibWeb/Layout/LineBoxFragment.h

@@ -6,14 +6,13 @@
 
 #pragma once
 
-#include <AK/Weakable.h>
 #include <LibGfx/Forward.h>
 #include <LibGfx/Rect.h>
 #include <LibWeb/Forward.h>
 
 namespace Web::Layout {
 
-class LineBoxFragment : public Weakable<LineBoxFragment> {
+class LineBoxFragment {
     friend class LineBox;
 
 public: