Linux: add a Credentials class to handle Linux capabilities.

(This is a re-land of https://chromiumcodereview.appspot.com/51113009/)

BUG=312380, 312572
TBR=jorgelo@chromium.org

Review URL: https://codereview.chromium.org/60513003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233027 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233027