Cleanup WaitpidWithTimeout

Cleanup WaitpidWithTimeout

This CL gives WaitpidWithTimeout() an API more similar to waitpid();
i.e., it returns the success state via the return value, and the
process's termination status via an out-parameter.  It also avoids
assuming that "status == -1" is an invalid status code, as POSIX makes
no guarantees about the representation of status values.

Lastly (and my original motivation), the code now avoids unnecessarily
relying on non-portable waitpid() behavior.  POSIX arguably requires
waitpid() to *not* modify the status value if it returns 0, but at least
OpenBSD still modifies status in this case, which then triggers a DCHECK
failure in base_unittests.

TEST="base_unittests --gtest_filter=ProcessUtilTest.DelayedTermination" on OpenBSD

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=275983

[brettw, 2014-06-09 16:57:19]

lgtm

[mdempsky, 2014-06-09 17:25:12]

The CQ bit was checked by mdempsky@chromium.org

[commit-bot: I haz the power, 2014-06-09 17:26:10]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mdempsky@chromium.org/319703007/1

[commit-bot: I haz the power, 2014-06-10 11:09:16]

Change committed as 275983