Add DropPrivileges()

Add DropPrivileges().

DropPrivileges() is used in exception_port_tool, so that when it is
installed as a setuid executable, it only uses elevated privileges to
obtain a task port for its -p option, and then relinquishes those
privileges.

It is difficult to provide a test for this function, because it must be
running setuid or setgid in order to do anything interesting. However,
the function contains its own CHECKs to verify that it behaves properly.

R=rsesek@chromium.org

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/49d7fdba9af9d465dc4eda940585523d810008e3

[Mark Mentovai, 2014-11-14 21:52:11]

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

[Robert Sesek, 2014-11-14 23:36:03]

LGTM

https://codereview.chromium.org/727053002/diff/20001/util/posix/drop_privileg...
File util/posix/drop_privileges.cc (right):

https://codereview.chromium.org/727053002/diff/20001/util/posix/drop_privileg...
util/posix/drop_privileges.cc:65: // CAP_SETUID and CAP_SETGID capabilities,
which may be granted to non-root
Arguably, this function should drop those capabilities.

[Mark Mentovai, 2014-11-14 23:44:29]

Committed patchset #2 (id:20001) manually as
49d7fdba9af9d465dc4eda940585523d810008e3 (presubmit successful).