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

net/log/file_net_log_observer.cc

 // 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.
 
 #include "net/log/file_net_log_observer.h"
 
 #include <limits>
 #include <memory>
 #include <queue>
 #include <set>
@@ skipping 279 matching lines @@
   }
   file_task_runner_->DeleteSoon(FROM_HERE, file_writer_.release());
 }
 
 void FileNetLogObserver::StartObserving(NetLog* net_log,
                                         NetLogCaptureMode capture_mode) {
   net_log->DeprecatedAddObserver(this, capture_mode);
 }
 
 void FileNetLogObserver::StopObserving(std::unique_ptr<base::Value> polled_data,
-                                       const base::Closure& callback) {
+                                       base::OnceClosure optional_callback) {
   net_log()->DeprecatedRemoveObserver(this);
 
-  file_task_runner_->PostTaskAndReply(
-      FROM_HERE,
+  base::OnceClosure bound_flush_then_stop =
       base::Bind(&FileNetLogObserver::FileWriter::FlushThenStop,
                  base::Unretained(file_writer_.get()), write_queue_,
-                 base::Passed(&polled_data)),
-      callback);
+                 base::Passed(&polled_data));
+
+  // Note that PostTaskAndReply() requires a non-null closure.
+  if (!optional_callback.is_null()) {
+    file_task_runner_->PostTaskAndReply(FROM_HERE,
+                                        std::move(bound_flush_then_stop),
+                                        std::move(optional_callback));
+  } else {
+    file_task_runner_->PostTask(FROM_HERE, std::move(bound_flush_then_stop));
+  }
 }
 
 void FileNetLogObserver::OnAddEntry(const NetLogEntry& entry) {
   std::unique_ptr<std::string> json(new std::string);
 
   // If |entry| cannot be converted to proper JSON, ignore it.
   if (!base::JSONWriter::Write(*entry.ToValue(), json.get()))
     return;
 
   size_t queue_size = write_queue_->AddEntryToQueue(std::move(json));
@@ skipping 232 matching lines @@
 void FileNetLogObserver::UnboundedFileWriter::DeleteAllFiles() {
   DCHECK(task_runner_->RunsTasksInCurrentSequence());
 
   // Reset |file_| to release the file handle so base::DeleteFile can
   // safely access it.
   file_.reset();
   base::DeleteFile(file_path_, false);
 }
 
 }  // namespace net