Replace linux_chromium_headless_dbg with linux_chromium_headless_rel

masters/master.tryserver.chromium.linux/slaves.cfg

Index: masters/master.tryserver.chromium.linux/slaves.cfg
diff --git a/masters/master.tryserver.chromium.linux/slaves.cfg b/masters/master.tryserver.chromium.linux/slaves.cfg
index 90d34ddf6f996800f7087306b4f27fd474a0f22c..ea0db4167f5388f6c575685ff4c7e5c87cbd7509 100644
--- a/masters/master.tryserver.chromium.linux/slaves.cfg
+++ b/masters/master.tryserver.chromium.linux/slaves.cfg
@@ -176,7 +176,7 @@ def linux():
     }
   ])
 
-  # Slaves for linux_chromium_headless_dbg.
+  # Slaves for linux_chromium_headless_rel.
   headless_slaves = [426, 497, 776, 864, 865, 872] + range(1069, 1098)
   for slave in headless_slaves:
     result.append({
@@ -184,9 +184,9 @@ def linux():
       'os': 'linux',
       'version': 'trusty',
       'bits': 64,
-      'builder': ['linux_chromium_headless_dbg'],
+      'builder': ['linux_chromium_headless_rel'],
       'hostname': 'slave%d-c4' % slave,
-      'pool': 'linux_chromium_headless_dbg',
+      'pool': 'linux_chromium_headless_rel',
     })
 
   return result