Add 'mac_x64' trybot to default set.

This trybot was recently added since we've run into
some failures only happening for Mac 64 Debug.

TBR=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/41969004

Cr-Commit-Position: refs/heads/master@{#8382}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8382 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 320b07a..81196c9 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -352,6 +352,7 @@
       'mac_asan',
       'mac_baremetal',
       'mac_rel',
+      'mac_x64',
       'mac_x64_rel',
   ] + mac_gn_bots
   win_gn_bots = [