Linux Sandbox: add Credentials::SupportsNewUserNS()

We need to be able to check for unprivileged namespace support without
actually unsharing the current namespace.

We add a method that creates a new process with a new user namespace and
see if this succeeds.

BUG=312380
NOTRY=true

Review URL: https://codereview.chromium.org/182453004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254433 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed