Add support for Chrome policy type that references external data

grit/format/policy_templates/writers/admx_writer.py

Index: grit/format/policy_templates/writers/admx_writer.py
===================================================================
--- grit/format/policy_templates/writers/admx_writer.py	(revision 139)
+++ grit/format/policy_templates/writers/admx_writer.py	(working copy)
@@ -253,6 +253,9 @@
     policies_elem = self._active_policies_elem
     policy_type = policy['type']
     policy_name = policy['name']
+    if policy_type == 'external':
+      # This type can only be set through cloud policy.
+      return
 
     attributes = {
       'name': name,