WebRTC: Updates a->b lab machines for tryserver android_dbg master

masters/master.tryserver.webrtc/slaves.cfg

 # 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.
 
 slaves = [
   # Linux.
   {
     'master': 'WebRTCTryServer',
     'os': 'linux',
     'version': 'trusty',
@@ skipping 30 matching lines @@
 ] + [
   # Linux bare-metal machines with Android devices.
   {
     'master': 'WebRTCTryServer',
     'os': 'linux',
     'version': 'trusty',
     'bits': '64',
     'builder': [
       'android_dbg',
     ],
-    'hostname': 'build%d-a4' % i,
-  } for i in range(77, 79)
+    'hostname': 'build%d-b4' % i,
+  } for i in range(141, 143)
 ] + [
   {
     'master': 'WebRTCTryServer',
     'os': 'linux',
     'version': 'trusty',
     'bits': '64',
     'builder': [
       'android_arm64_rel',
     ],
     'hostname': 'build140-b4',
@@ skipping 107 matching lines @@
     'master': 'WebRTCTryServer',
     'os': 'win',
     'version': 'win7',
     'bits': '64',
     'builder': [
       'win_baremetal',
     ],
     'hostname': hostname,
   } for hostname in ['build143-b4', 'build139-b4']
 ]