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/831373002
Cr-Commit-Position: refs/heads/master@{#310327}
10 files changed
