Certificate Transparency: Ensure no observers in STHDistributor d'tor.

net/cert/sth_distributor.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_CERT_STH_DISTRIBUTOR_H_
 #define NET_CERT_STH_DISTRIBUTOR_H_
 
 #include <vector>
 
 #include "base/observer_list.h"
@@ skipping 28 matching lines @@
 
   // Unregisters |observer|, which must have been previously
   // registered via RegisterObserver()
   void UnregisterObserver(STHObserver* observer) override;
 
  private:
   // STHs from logs, one for each log.
   std::vector<SignedTreeHead> observed_sths_;
 
   // The observers for new STH notifications.
-  base::ObserverList<STHObserver> observer_list_;
+  base::ObserverList<STHObserver, true> observer_list_;
 };
 
 }  // namespace ct
 
 }  // namespace net
 
 #endif  // NET_CERT_STH_DISTRIBUTOR_H_