Linux sandbox: move CurrentProcessHasOpenDirectories

Move CurrentProcessHasOpenDirectories() to the Credentials class and rename
it to HasOpenDirectory().
Also add some unittests.

This is a re-land of https://codereview.chromium.org/85403011/.

BUG=312380
R=jorgelo@chromium.org, mmoss@google.com

Review URL: https://codereview.chromium.org/88243003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237390 0039d316-1c4b-4281-b951-d872f2087c98
7 files changed