Tweak the way we manage connected android devices in run-webkit-tests.

By sorting the list of device serial numbers, we can ensure that, on
a given machine w/ a given set of connected devices, we always have
the same mapping of worker number to device.

R=abarth@chromium.org, peter@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/23469011

git-svn-id: svn://svn.chromium.org/blink/trunk@157148 bbb929c8-8fbe-4397-9dbb-9b2b20218538