shill: Ensure metrics dir existence/ownership/perms

Ensure existence/ownership/perms every time shill-pre-start.sh runs,
since we have another Upstart script that needs to use/potentially
create that dir (but omits the change ownership/perms step).

BUG=chromium:649417
TEST=code on DUT does same thing as before

Change-Id: Ic63f680ebd30d25baef062a6ae16dc451be4bd63
Reviewed-on: https://chromium-review.googlesource.com/1204652
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Micah Morton <mortonm@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>