Move ForkWithFlags from sandbox/ to base/ and plug it into LaunchProcess.

ForkWithFlags is a wrapper around the clone syscall that uses the libc clone
wrapper, and thus updates the libc's pid cache if it has one (using sys_clone
directly does not update the pid cache, so getpid may return an incorrect
result in the child). This exposes the ability to set clone flags, which is
needed to use Linux namespaces.

BUG=312380

Review URL: https://codereview.chromium.org/840893003

Cr-Commit-Position: refs/heads/master@{#311356}
10 files changed