[Android] Raise NoDevicesError in DeviceUtils.parallel.

build/android/pylib/device/device_utils_test.py

Index: build/android/pylib/device/device_utils_test.py
diff --git a/build/android/pylib/device/device_utils_test.py b/build/android/pylib/device/device_utils_test.py
index 66db8cde292a2f1a1c8be0abeb99e854579bfb1d..3f4ef597b6dedbee91e3d535d635142841775775 100755
--- a/build/android/pylib/device/device_utils_test.py
+++ b/build/android/pylib/device/device_utils_test.py
@@ -1404,6 +1404,12 @@ class DeviceUtilsParallelTest(mock_calls.TestCase):
           and serial == str(device),
           'Expected a DeviceUtils object with serial %s' % serial)
 
+  def testParallel_noDevices(self):
+    with self.assertCall(
+        mock.call.pylib.device.adb_wrapper.AdbWrapper.GetDevices(), []):
+      with self.assertRaises(device_errors.NoDevicesError):
+        device_utils.DeviceUtils.parallel()
+
 
 if __name__ == '__main__':
   logging.getLogger().setLevel(logging.DEBUG)