cgroups: partial revert of /sys/fs/cgroup/cpuset backport

These older branches don't have osutils.IterateMountPoints, so
revert the change back to using the simpler/dumber _FileContains.

BUG=chromium:883000
TEST=None

Change-Id: I6756a20cf2c760547c7d53f5ab5f71054361e0af
(cherry picked from commit 0588e68ae1ecdac13972accfe415a87a9f6d2b9a)
Reviewed-on: https://chromium-review.googlesource.com/1223744
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed