Prevent sandboxed processes from opening each other

      TBR=brettw
BUG=117627
BUG=119150
TEST=sbox_validation_tests
Review URL: https://chromiumcodereview.appspot.com/9716027

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@132477 0039d316-1c4b-4281-b951-d872f2087c98