Stop using chrome-sandbox to determine real pids

Stop using chrome-sandbox to determine real pids

Previously, when the zygote forked a child process, the parent would
send an IPC to the browser process, which then used chrome-sandbox to
do some procfs trickery to correlate PIDs across PID namespaces.

However, since Linux 3.0, PIDs provided by the kernel (e.g., by
SCM_CREDENTIALS) are automatically translated to the receiver's PID
namespace.  To take advantage of this, this CL makes two changes:

1. Add a new sandbox IPC request to send back the caller's PID as seen
by the browser PID namespace.

2. Switch roles so the fork child now issues the sandbox IPC to
determine its own PID, and the browser writes the child's PID over the
pipe for the parent to read.

BUG=357670

[mdempsky, 2014-04-25 04:42:23]

This should be ready for review now, but there's going to be some
overlap/conflicts with crrev.com/258543006 (Change UnixDomainSocket::RecvMsg to
return ScopedVector<base::ScopedFD>), so it's fine by me if you'd rather wait
until I've submitted that one and fixed up the conflicts here.

[mdempsky, 2014-04-25 23:48:59]

Hold off on reviewing this.  I think I can simplify things so that:
  1. We get rid of the extra sandbox IPC that nobody likes.
  2. I can split it into two logically distinct CLs that are easier to review
separately and in parallel.