URLFetcher won't call delegate until file is closed.

net/base/file_stream_metrics.h

 // Copyright (c) 2012 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.
 
 // File error statistics gathering.
 
 #ifndef NET_BASE_FILE_STREAM_METRICS_H_
 #define NET_BASE_FILE_STREAM_METRICS_H_
 
 namespace net {
 
 enum FileErrorSource {
   FILE_ERROR_SOURCE_OPEN = 0,
   FILE_ERROR_SOURCE_WRITE,
   FILE_ERROR_SOURCE_READ,
   FILE_ERROR_SOURCE_SEEK,
   FILE_ERROR_SOURCE_FLUSH,
   FILE_ERROR_SOURCE_SET_EOF,
   FILE_ERROR_SOURCE_GET_SIZE,
+  FILE_ERROR_SOURCE_CLOSE,
   FILE_ERROR_SOURCE_COUNT,
 };
 
 // UMA error statistics gathering.
 // Put the error value into a bucket.
 int GetFileErrorUmaBucket(int error);
 
 // The largest bucket number, plus 1.
 int MaxFileErrorUmaBucket();
 
 // The highest error value we want to individually report.
 int MaxFileErrorUmaValue();
 
 // |error| is a platform-specific error (Windows or Posix).
 // |source| indicates the operation that resulted in the error.
 // |record| is a flag indicating that we are interested in this error.
 void RecordFileError(int error, FileErrorSource source, bool record);
 
 // Gets a description for the source of a file error.
 const char* GetFileErrorSourceName(FileErrorSource source);
 
 }  // namespace net
 
 #endif  // NET_BASE_FILE_STREAM_METRICS_H_