It is possible that power LED goes off while AC still connected. Let's
set power LED periodically to aviod this problem.
BUG=chrome-os-partner:10386
TEST=Disconnect power LED while leave AC connected. Check LED goes off.
Connect LED again and check it comes back after few seconds.
Change-Id: I2a199446be5da772af8027b735b9f431f697bacd
Reviewed-on: https://gerrit.chromium.org/gerrit/29403
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Ready: Vic Yang <victoryang@chromium.org>
Tested-by: Vic Yang <victoryang@chromium.org>