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Since there is now a class SingleDebugDaemonLogSource, the old
name "SingleLogSource" is ambiguous. Rename it to indicate that the
logs come from a log file.

Also refactored the use of SingleLogSource in feedbackPrivate API:
- Use the base interface class SystemLogsSource instead, in
  LogSourceAccessManager.
- Convert from feedback_private::LogSource directly to a
  SystemLogSource in SingleLogSourceFactory. Conversion of enums is
  internal to SingleLogSourceFactory.

BUG=733846

Review-Url: https://codereview.chromium.org/2956513004
Cr-Commit-Position: refs/heads/master@{#485283}