To run task_for_pid() on child processes, we need the child task port for
security reasons. This port can be obtained via a Mach IPC exchange.
This is what GeckoChildProcessHost::GetChildTask() provides, so we use it
in cocoa's version of GetProcInfo()
Differential Revision: https://phabricator.services.mozilla.com/D25927
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extra : moz-landing-system : lando