WebRTC: Add more VMs + dedicate two for presubmit bot.

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 23 matching lines @@
       'linux_gyp_rel',
       'linux_libfuzzer_rel',
       'linux_arm',
       'linux_asan',
       'linux_memcheck',
       'linux_msan',
       'linux_swarming',
       'linux_tsan2',
       'linux_ubsan',
       'linux_ubsan_vptr',
+    ],
+    'hostname': 'slave%d-c4' % i,
+  } for i in range(719, 726) + range(837, 841) + range(1210, 1220)
+] + [
+  {
+    'master': 'WebRTCTryServer',
+    'os': 'linux',
+    'version': 'trusty',
+    'bits': '64',
+    'builder': [
       'presubmit',
     ],
     'hostname': 'slave%d-c4' % i,
-  } for i in range(719, 726) + range(777, 779) + range(837, 841)
+  } for i in range(777, 779)
 ] + [
   # Linux bare-metal machines with audio devices and headsets.
   {
     'master': 'WebRTCTryServer',
     'os': 'linux',
     'version': 'trusty',
     'bits': '64',
     'builder': [
       'linux_baremetal',
     ],
@@ skipping 86 matching lines @@
     'master': 'WebRTCTryServer',
     'os': 'win',
     'version': 'win7',
     'bits': '64',
     'builder': [
       'win_baremetal',
     ],
     'hostname': hostname,
   } for hostname in ['build143-b4', 'build139-b4']
 ]