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

masters/master.tryserver.webrtc/slaves.cfg

Index: masters/master.tryserver.webrtc/slaves.cfg
diff --git a/masters/master.tryserver.webrtc/slaves.cfg b/masters/master.tryserver.webrtc/slaves.cfg
index b4fd4034af7a1388b1720d4ea4a851ab1d6f813f..121b35b03f2bd6478a013e8ac5db488237c7004f 100644
--- a/masters/master.tryserver.webrtc/slaves.cfg
+++ b/masters/master.tryserver.webrtc/slaves.cfg
@@ -41,10 +41,20 @@ slaves = [
       '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.
   {