[base] Fix wrong ThreadedList append for empty lists allocated on the stack.

Change-Id: Ieae88990f3d960c13a2bafc223e12061e994fce0
Reviewed-on: https://chromium-review.googlesource.com/c/1270580Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Florian Sattler <sattlerf@google.com>
Cr-Commit-Position: refs/heads/master@{#56474}

src/base/threaded-list.h

@@ -69,6 +69,8 @@ class ThreadedListBase final : public BaseClass {
   }
 
   void Append(ThreadedListBase&& list) {
+    if (list.is_empty()) return;
+
     *tail_ = list.head_;
     tail_ = list.tail_;
     list.Clear();

test/unittests/base/threaded-list-unittest.cc

@@ -166,6 +166,23 @@ TEST_F(ThreadedListTest, Append) {
   CHECK_EQ(list.end(), initial_extra_list_end);
 }
 
+TEST_F(ThreadedListTest, AppendOutOfScope) {
+  ThreadedListTestNode local_extra_test_node_0;
+  CHECK_EQ(list.LengthForTest(), 5);
+  {
+    ThreadedList<ThreadedListTestNode, ThreadedListTestNode::OtherTraits>
+        scoped_extra_test_list;
+
+    list.Append(std::move(scoped_extra_test_list));
+  }
+  list.Add(&local_extra_test_node_0);
+
+  list.Verify();
+  CHECK_EQ(list.LengthForTest(), 6);
+  CHECK_EQ(list.AtForTest(4), &nodes[4]);
+  CHECK_EQ(list.AtForTest(5), &local_extra_test_node_0);
+}
+
 TEST_F(ThreadedListTest, Prepend) {
   CHECK_EQ(list.LengthForTest(), 5);
   list.Prepend(std::move(extra_test_list));