Make rebaselining work when the test passes on all the bots.

Right now it hits an assert in run_in_parallel that the command
list is not empty. This change makes the mock have that assert
and avoids calling run_in_parallel if the list is empty.

Review URL: https://chromiumcodereview.appspot.com/23921002

git-svn-id: svn://svn.chromium.org/blink/trunk@157487 bbb929c8-8fbe-4397-9dbb-9b2b20218538