Use FileNetLogObserver for implementing --log-net-log.

net/log/file_net_log_observer.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_LOG_FILE_NET_LOG_OBSERVER_H_
 #define NET_LOG_FILE_NET_LOG_OBSERVER_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 24 matching lines @@
 //
 // The user is able to specify an approximate maximum cumulative size for the
 // netlog files and the observer overwrites old events when the maximum file
 // size is reached.
 //
 // Unbounded mode:
 // The entire JSON object is put into one file. There is no size limit to how
 // large this file can grow; all events added will be written to the file.
 //
 // The consumer must call StartObserving before calling StopObserving, and must
-// call each method exactly once in the lifetime of the observer. StartObserving
-// and StopObserving must be called on the same thread, but there is no
-// restriction on which thread is used.
+// call each method exactly once in the lifetime of the observer.
 class NET_EXPORT FileNetLogObserver : public NetLog::ThreadSafeObserver {
  public:
   // Creates a FileNetLogObserver in bounded mode.
   //
   // |directory| is the directory where the log files will be written to.
   //
   // |max_total_size| is the approximate limit on the cumulative size of all
   // netlog files.
   //
   // |total_num_files| sets the total number of event files that are used to
@@ skipping 25 matching lines @@
   void StartObserving(NetLog* net_log, NetLogCaptureMode capture_mode);
 
   // Stops observing net_log() and closes the output file(s). Must be called
   // after StartObserving. Should be called before destruction of the
   // FileNetLogObserver and the NetLog, or the NetLog files will be deleted when
   // the observer is destroyed.
   //
   // |polled_data| is an optional argument used to add additional network stack
   // state to the log.
   //
-  // |callback| will be run on whichever thread StopObserving() was called on
-  // once all file writing is complete and the netlog files can be accessed
-  // safely.
+  // If non-null, |optional_callback| will be run on whichever thread
+  // StopObserving() was called on once all file writing is complete and the
+  // netlog files can be accessed safely.
   void StopObserving(std::unique_ptr<base::Value> polled_data,
-                     const base::Closure& callback);
+                     base::OnceClosure optional_callback);
 
   // NetLog::ThreadSafeObserver
   void OnAddEntry(const NetLogEntry& entry) override;
 
  private:
   class WriteQueue;
   class FileWriter;
   class BoundedFileWriter;
   class UnboundedFileWriter;
 
@@ skipping 17 matching lines @@
   // deletion is deferred until outstanding tasks on |file_task_runner_| have
   // finished (since it is posted using base::Unretained()).
   std::unique_ptr<FileWriter> file_writer_;
 
   DISALLOW_COPY_AND_ASSIGN(FileNetLogObserver);
 };
 
 }  // namespace net
 
 #endif  // NET_LOG_FILE_NET_LOG_OBSERVER_H_