[Mac] Terminate child processes cleanly if they fail to Mach rendezvous.

If the browser terminates shortly after launching a child process, the
child will fail to look up the bootstrap server the browser was exposing
in order to perform Mach port rendezvous. When this happens, rather than
crashing due to follow-on failures, simply terminate the process
cleanly.

This situation occurs when when launching Content Shell with
`--run-web-tests` but then not specifying a test case (so the browser
exits cleanly right after launching child processes). It also appears to
happen infrequently in production when the browser crashes. The crash
reports that are generated from the child processes in this case are
uninteresting, so exiting cleanly is a better solution that generating
uninteresting reports.

Bug: 980696
Change-Id: Id4733482b63c0d33931f25923744a103072dd5a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1695961
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676481}