Random cleanup for c/b/cros/extensions/file_manager.

- Remove unused #include's.
- Remove obsolete comments.
- Prefer not to access error_ protected member directly.

BUG=none

Review URL: https://codereview.chromium.org/141473004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247666 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247666