Fix return value of new slave-distribution function

scripts/master/master_utils.py

 # Copyright (c) 2012 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.
 
 import collections
 import os
 import random
 import re
 import sys
 
@@ skipping 687 matching lines @@
     elif None in prefs:
       # Second choice: Slaves that don't prefer any builders.
       key = None
     else:
       # Third choice: Slaves that prefer other builders but have largest
       # capacity left. If several groups of slaves with equal capacity exist,
       # one group will be chosen arbitrarily, the actual slave will be chosen
       # randomly.
       key = prefs.most_common()[0][0]
     return self._choice(
-        s for s in slave_builders
-        if s.slave.properties.getProperty('preferred_builder') == key)
+        [s for s in slave_builders
+         if s.slave.properties.getProperty('preferred_builder') == key])
 
 
 def SetMasterProcessName():
   """Sets the name of this process to the name of the master.  Linux only."""
 
   if sys.platform != 'linux2':
     return
 
   command_line.set_command_line("master: %s" % GetMastername())