Add landmine for a clobber on mac bots.

build/get_landmines.py

 #!/usr/bin/env python
 # Copyright 2013 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.
 
 """
 This file emits the list of reasons why a particular build needs to be clobbered
 (or a list of 'landmines').
 """
 
@@ skipping 59 matching lines @@
   print 'Clobber to fix GN not picking up increased ID range (crbug.com/444902)'
   print 'Remove NaCl toolchains from the output dir (crbug.com/456902)'
   if platform() == 'ios':
     print 'Clobber iOS to workaround Xcode deps bug (crbug.com/485435)'
   if platform() == 'win':
     print 'Clobber to delete stale generated files (crbug.com/510086)'
   if platform() == 'android' and gyp_defines().get('target_arch') == 'arm64':
     print 'Clobber to support new location/infra for chrome_sync_shell_apk'
   if platform() == 'mac':
     print 'Clobber to get rid of evil libsqlite3.dylib (crbug.com/526208)'
+  if platform() == 'mac':
+    print 'Clobber to remove libsystem.dylib. See crbug.com/620075'
 
 
 def main():
   print_landmines()
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())