bootstrap_look_up() “successfully” returns MACH_PORT_DEAD about half of
the time on 10.12.1 16B2657 (xnu-3789.21.4). Replace that with
MACH_PORT_NULL in the BootstrapLookUp() wrapper that all callers are
already routed through.
BUG=crashpad:139
TEST=crashpad_util_test MachExtensions.BootstrapCheckInAndLookUp
Change-Id: I9a39b709add5ca7e64bb5b970ed6ba3fdfd1d47a
Reviewed-on: https://chromium-review.googlesource.com/409671
Reviewed-by: Robert Sesek <rsesek@chromium.org>