Use only the actually available devices in run-webkit-tests --android.

On Android, we default to using the number of devices 'adb devices' tells us
are available. However, during setup_test_run() and check_build() we may
discover that some of them don't actually function. Prior to this patch,
we had no way to tell the calling code how many were actually usable
and hence how many workers we should use.

R=abarth@chromium.org, peter@chromium.org
BUG=305040

Review URL: https://codereview.chromium.org/84173002

git-svn-id: svn://svn.chromium.org/blink/trunk@163668 bbb929c8-8fbe-4397-9dbb-9b2b20218538