[Android] Fix DeviceUtils.__str__ when no serial is provided.

build/android/pylib/device/device_utils_test.py

 #!/usr/bin/env python
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """
 Unit tests for the contents of device_utils.py (mostly DeviceUtils).
 """
 
 # pylint: disable=C0321
@@ skipping 1466 matching lines @@
         "adb -s 0123456789abcdef shell 'showmap 4321'",
         'cannot open /proc/4321/smaps: No such file or directory\r\n'):
       self.assertEqual({}, self.device.GetMemoryUsageForPid(4321))
 
 
 class DeviceUtilsStrTest(DeviceUtilsOldImplTest):
   def testStr_noAdbCalls(self):
     with self.assertNoAdbCalls():
       self.assertEqual('0123456789abcdef', str(self.device))
 
+  def testStr_noSerial(self):
+    self.device = device_utils.DeviceUtils(None)
+    with self.assertCalls('adb  get-serialno', '0123456789abcdef'):
+      self.assertEqual('0123456789abcdef', str(self.device))
+
+  def testStr_noSerial_noDevices(self):
+    self.device = device_utils.DeviceUtils(None)
+    with self.assertCalls('adb  get-serialno', 'unknown'), (
+         self.assertRaises(device_errors.NoDevicesError)):
+      str(self.device)
+
 
 if __name__ == '__main__':
   logging.getLogger().setLevel(logging.DEBUG)
   unittest.main(verbosity=2)