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

build/android/pylib/device/device_utils.py

 # 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.
 
 """Provides a variety of device interactions based on adb.
 
 Eventually, this will be based on adb_wrapper.
 """
 # pylint: disable=W0613
 
@@ skipping 772 matching lines @@
         - A dict containing the overall memory usage statistics for the PID.
         - A dict containing memory usage statistics broken down by mapping.
 
     Raises:
       CommandTimeoutError on timeout.
     """
     return self.old_interface.GetMemoryUsageForPid(pid)
 
   def __str__(self):
     """Returns the device serial."""
-    return self.old_interface.GetDevice()
+    s = self.old_interface.GetDevice()
+    if not s:
+      s = self.old_interface.Adb().GetSerialNumber()
+      if s == 'unknown':
+        raise device_errors.NoDevicesError()
+    return s
 
   @staticmethod
   def parallel(devices=None, async=False):
     """Creates a Parallelizer to operate over the provided list of devices.
 
     If |devices| is either |None| or an empty list, the Parallelizer will
     operate over all attached devices.
 
     Args:
       devices: A list of either DeviceUtils instances or objects from
                from which DeviceUtils instances can be constructed. If None,
                all attached devices will be used.
       async: If true, returns a Parallelizer that runs operations
              asynchronously.
 
     Returns:
       A Parallelizer operating over |devices|.
     """
     if not devices or len(devices) == 0:
       devices = pylib.android_commands.GetAttachedDevices()
     parallelizer_type = (parallelizer.Parallelizer if async
                          else parallelizer.SyncParallelizer)
     return parallelizer_type([
         d if isinstance(d, DeviceUtils) else DeviceUtils(d)
         for d in devices])