LibJS: Put zombie cell tracking code behind a compile-time flag

Since this is a debug-only feature, let's not have it impact GC marking
performance when you don't need it.

Userland/Libraries/LibJS/Heap/Cell.h

@@ -24,12 +24,18 @@ public:
     bool is_marked() const { return m_mark; }
     void set_marked(bool b) { m_mark = b; }
 
-    virtual void did_become_zombie() { }
+#ifdef JS_TRACK_ZOMBIE_CELLS
+    virtual void did_become_zombie()
+    {
+    }
+#endif
 
     enum class State {
         Live,
         Dead,
+#ifdef JS_TRACK_ZOMBIE_CELLS
         Zombie,
+#endif
     };
 
     State state() const { return m_state; }