[login_manager] Be more aggressive about de-registering child-exit handlers

Perhaps doing this earlier in the shutdown process will make 14840 go away.  Either way, there still exists something of a race here.  Try to close it.

(port of http://gerrit.chromium.org/gerrit/1074 to the R12 branch)

TEST=unit tests, suite_Smoke

Change-Id: I5f8f5c524c8a8854cde0d4f183901fc0a23bf0df
Reviewed-on: http://gerrit.chromium.org/gerrit/1081
Tested-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: Zelidrag Hornung <zelidrag@chromium.org>
2 files changed