WebRTC: Update to new machines after a->b lab move.

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 217df2e1959dbcb8985bb3140943fe3092f3fc4b..8f3306b0d7dea64569d986b0cd7373723b26dcfe 100644
--- a/masters/master.tryserver.webrtc/slaves.cfg
+++ b/masters/master.tryserver.webrtc/slaves.cfg
@@ -59,7 +59,7 @@ slaves = [
     'builder': [
       'android_arm64_rel',
     ],
-    'hostname': 'build204-a4',
+    'hostname': 'build140-b4',
   },
   {
     'master': 'WebRTCTryServer',
@@ -93,7 +93,7 @@ slaves = [
       'linux_baremetal',
     ],
     'hostname': hostname,
-  } for hostname in ['build22-a4', 'build138-b4']
+  } for hostname in ['build144-b4', 'build138-b4']
 ] + [
   # Mac.
   {
@@ -175,5 +175,5 @@ slaves = [
       'win_baremetal',
     ],
     'hostname': hostname,
-  } for hostname in ['build23-a4', 'build139-b4']
+  } for hostname in ['build143-b4', 'build139-b4']
 ]