Namespace sandbox: add important security checks

When engaging the namespace sandbox, add important checks that the process
is single threaded and has no directory file descriptor open.

As part of this change, move the function engaging the namespace
sandbox from the Zygote to the LinuxSandbox class.

BUG=457377, 312380

Review URL: https://codereview.chromium.org/915823002

Cr-Commit-Position: refs/heads/master@{#315932}
8 files changed