Add TaskForPID()

Add TaskForPID().

This also transitions exception_port_tool to use TaskForPID(), so that
it can be safely used as a setuid executable without giving permission
to operate on any process on the system.

It is difficult to provide a test for this function, because it must be
running setuid root in order to do anything interesting.

R=rsesek@chromium.org

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/de3c46c6b396e6600fcdb04146586c1962b96d00

[Mark Mentovai, 2014-11-14 21:52:02]

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

[Mark Mentovai, 2014-11-14 21:52:03]

This depends on https://codereview.chromium.org/727973002/.

[Robert Sesek, 2014-11-14 22:50:15]

LGTM

https://codereview.chromium.org/728973002/diff/1/util/mach/task_for_pid.cc
File util/mach/task_for_pid.cc (right):

https://codereview.chromium.org/728973002/diff/1/util/mach/task_for_pid.cc#ne...
util/mach/task_for_pid.cc:48: // different_groups will only contain elements of
groups not present in
difference

[Mark Mentovai, 2014-11-14 22:56:22]

Committed patchset #3 (id:40001) manually as
de3c46c6b396e6600fcdb04146586c1962b96d00 (presubmit successful).