Bug 803688 - Re-land backed out part of bug 803688 now that we have bug 805207 fixed.

This changeset reverts the partial-backout cset 5c82f5a5e90d, taking us back to the state we were in at bug 803688's initial landing, rev 592c3465a742.

mfbt/LinkedList.h

@@ -254,6 +254,10 @@ class LinkedList
   public:
     LinkedList() : sentinel(LinkedListElement<T>::NODE_KIND_SENTINEL) { }
 
+    ~LinkedList() {
+      MOZ_ASSERT(isEmpty());
+    }
+
     /*
      * Add elem to the front of the list.
      */