When updating local roles, call createExpressionContext only if required

This makes 'updateLocalRolesOnDocument' 2.5 times faster (according to
hotshot) if there is no security defined.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@30349 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/ERP5Type.py

@@ -102,8 +102,10 @@ class LocalRoleAssignorMixIn(object):
     security.declarePrivate('getFilteredRoleListFor')
     def getFilteredRoleListFor(self, ob=None):
       """Return all role generators applicable to the object."""
-      ec = createExpressionContext(ob)
+      ec = None # createExpressionContext is slow so we call it only if needed
       for role in self.getRoleInformationList():
+        if ec is None:
+          ec = createExpressionContext(ob)
         if role.testCondition(ec):
           yield role