Linux: add a Credentials class to handle Linux capabilities.

BUG=312380
R=jorgelo@chromium.org

Review URL: https://codereview.chromium.org/51113009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232280 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232280