MachMessageServer: invert the request buffer allocation logic

MachMessageServer: invert the request buffer allocation logic.

The existing implementation used the same logic as is found in
mach_msg_server(), but that logic seems incorrect. When the caller wants
to retry a mach_msg() receive of a too-large message that returns
MACH_RCV_TOO_LARGE, there’s no harm in attempting the receive with a
larger buffer initially. On the other hand, if the caller does not want
to retry such mach_msg() receive attempts, it’s an indication that the
caller is expecting to be intolerant of too-large messages, and there’s
no need to attempt the receive with a buffer any larger than requested.

TEST=util_test
R=rsesek@chromium.org

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/a68594234262beee7c0cc2ad9f73b3c08264f30f

[Mark Mentovai, 2014-11-25 05:26:07]

mark@chromium.org changed reviewers:
+ rsesek@chromium.org

[Robert Sesek, 2014-11-25 19:34:05]

LGTM

[Mark Mentovai, 2014-11-25 20:04:34]

Committed patchset #3 (id:40001) manually as
a68594234262beee7c0cc2ad9f73b3c08264f30f (presubmit successful).