[Chromoting] Allow ProcessStatsSender to receive nullptr ProcessStatsAgent

remoting/host/process_stats_sender.h

 // Copyright 2017 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 REMOTING_HOST_PROCESS_STATS_SENDER_H_
 #define REMOTING_HOST_PROCESS_STATS_SENDER_H_
 
 #include <initializer_list>
 #include <vector>
 
@@ skipping 19 matching lines @@
   // and |agents|. They must outlive the ProcessStatsSender object.
   ProcessStatsSender(protocol::ProcessStatsStub* host_stats_stub,
                      base::TimeDelta interval,
                      std::initializer_list<ProcessStatsAgent*> agents);
 
   ~ProcessStatsSender();
  private:
   void ReportUsage();
 
   protocol::ProcessStatsStub* const host_stats_stub_;
-  std::vector<ProcessStatsAgent*> agents_;
+  const std::vector<ProcessStatsAgent*> agents_;
   base::RepeatingTimer timer_;
   const base::ThreadChecker thread_checker_;
 };
 
 }  // namespace remoting
 
 #endif  // REMOTING_HOST_PROCESS_STATS_SENDER_H_