Add ListContainer::swap

cc/base/list_container.h

Index: cc/base/list_container.h
diff --git a/cc/base/list_container.h b/cc/base/list_container.h
index 5463ca2066873bef445acb6a66bcbf084db2bb37..a9444cd03aa08d1e1192e625198e8222dae0ce36 100644
--- a/cc/base/list_container.h
+++ b/cc/base/list_container.h
@@ -288,6 +288,11 @@ class ListContainer : public ListContainerBase {
     return new (*at) DerivedElementType();
   }
 
+  template <typename DerivedElementType>
+  void swap(ListContainer<DerivedElementType>& other) {
+    data_.swap(other.data_);
+  }
+
   // Appends a new item without copying. The original item will not be
   // destructed and will be replaced with a new DerivedElementType. The
   // DerivedElementType does not have to match the moved type as a full block