Extract NetErrorToPlatformFileError from file_writer_delegate to file_system_util.

This is the preparation to share the utility function with other modules.

BUG=279278
TEST=Ran content_unittests
R=kinuko@chromium.org, tzik@chromium.org

Review URL: https://codereview.chromium.org/24300003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225143 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225143