Cleanup in {test_,}file_util_posix.cc

      - DCHECK_EQ instead of DCHECK(foo == bar)
- more detailed log error messages
- add a comment why we use 0777 permissions for new dirs (umask should produce sane permissions)

Review URL: http://codereview.chromium.org/40162

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@11105 0039d316-1c4b-4281-b951-d872f2087c98