Refactor SingleLogSource for better test coverage

chrome/browser/chromeos/system_logs/single_log_source.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 CHROME_BROWSER_CHROMEOS_SYSTEM_LOGS_SINGLE_LOG_SOURCE_H_
 #define CHROME_BROWSER_CHROMEOS_SYSTEM_LOGS_SINGLE_LOG_SOURCE_H_
 
 #include <stddef.h>
 
 #include "base/files/file.h"
@@ skipping 19 matching lines @@
 
   // system_logs::SystemLogsSource:
   void Fetch(const SysLogsSourceCallback& callback) override;
 
  private:
   friend class SingleLogSourceTest;
 
   // Reads all available content from |file_| that has not already been read.
   void ReadFile(SystemLogsResponse* result);
 
+  // Path to system log file directory.
+  base::FilePath log_file_dir_path_;
+
   // Keeps track of how much data has been read from |file_|.
   size_t num_bytes_read_;
 
   // Handle for reading the log file that is source of logging data.
   base::File file_;
 
   // For removing PII from log results.
   feedback::AnonymizerTool anonymizer_;
 
   base::WeakPtrFactory<SingleLogSource> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SingleLogSource);
 };
 
 }  // namespace system_logs
 
 #endif  // CHROME_BROWSER_CHROMEOS_SYSTEM_LOGS_SINGLE_LOG_SOURCE_H_