Use a bootstrap subset port for the inspector

Use a bootstrap subset port for the inspector, tying the subset to the
lifetime of the task to be monitored, the invoking task. This allows the
bootstrap server (in launchd) to automatically clean up the Mach server
registration when the task being monitored exits, avoiding leaks of
com.Breakpad.Inspector(pid) ports in "launchctl bslist".

BUG=chromium:28547
TEST=Handler should still crash catches, but inspector ports should no longer
     show up in "launchctl bslist". They should show up under a subset port in
     "launchctl bstree" instead. "launchctl bstree" must be invoked as root.

Committed r842.

[Mark Mentovai]

TBR. Committed Breakpad r842.

[mochalatte]

LGTM