Refactor SingleLogSource for better test coverage

chrome/browser/chromeos/system_logs/single_log_source.h

Index: chrome/browser/chromeos/system_logs/single_log_source.h
diff --git a/chrome/browser/chromeos/system_logs/single_log_source.h b/chrome/browser/chromeos/system_logs/single_log_source.h
index adca49cf41c6a9144eb1250731a683a9fa8dfcbc..2ffe730d9a7a59eb2910de1ab893397b4580d238 100644
--- a/chrome/browser/chromeos/system_logs/single_log_source.h
+++ b/chrome/browser/chromeos/system_logs/single_log_source.h
@@ -37,6 +37,9 @@ class SingleLogSource : public SystemLogsSource {
   // 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_;