This adds a pipe going in the other direction (parent to child).
Initially, I didn’t think this was necessary, but it turned out to be
needed for ProcessReader. Having the child wait on a pipe read is the
easiest way to keep it alive until the parent is done with it.
This also tests MachMultiprocess::ChildPID() in
mach_multiprocess_test.cc.
Both of these fell out of https://codereview.chromium.org/491963002/.
TEST=util_test MachMultiprocess.MachMultiprocess
R=rsesek@chromium.org
Review URL: https://codereview.chromium.org/491363002