LibJS: Make WeakContainer pruning do less work

Instead of iterating *all* swept cells when pruning weak containers,
only iterate the cells actually *in* the container.

Also, instead of compiling a list of all swept cells, we can simply
check the Cell::state() flag to know if something should be pruned.

Userland/Libraries/LibJS/Runtime/WeakContainer.h

@@ -15,7 +15,7 @@ public:
     explicit WeakContainer(Heap&);
     virtual ~WeakContainer();
 
-    virtual void remove_swept_cells(Badge<Heap>, Span<Cell*>) = 0;
+    virtual void remove_dead_cells(Badge<Heap>) = 0;
 
 protected:
     void deregister();