Mark PageLoad UKM as singular.

tools/metrics/ukm/model.py

 # Copyright 2017 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.
 # """Model objects for ukm.xml contents."""
 
 import os
 import sys
 
 sys.path.append(os.path.join(os.path.dirname(__file__), '..', 'common'))
 import models
@@ skipping 10 matching lines @@
     attributes=[('name', unicode)],
     children=[
         models.ChildType('obsolete', _OBSOLETE_TYPE, False),
         models.ChildType('owners', _OWNER_TYPE, True),
         models.ChildType('summary', _SUMMARY_TYPE, False),
     ])
 
 _EVENT_TYPE =  models.ObjectNodeType(
     'event',
     alphabetization=('metric', _LOWERCASE_NAME_FN),
-    attributes=[('name', unicode)],
+    attributes=[('name', unicode), ('singular', bool)],
     extra_newlines=(1, 1, 1),
     children=[
         models.ChildType('obsolete', _OBSOLETE_TYPE, False),
         models.ChildType('owners', _OWNER_TYPE, True),
         models.ChildType('summary', _SUMMARY_TYPE, False),
         models.ChildType('metrics', _METRIC_TYPE, True),
     ])
 
 _UKM_CONFIGURATION_TYPE = models.ObjectNodeType(
     'ukm-configuration',
@@ skipping 10 matching lines @@
 
   Args:
     original_xml: A string containing the original xml file contents.
 
   Returns:
     A pretty-printed xml string, or None if the config contains errors.
   """
   config = UKM_XML_TYPE.Parse(original_xml)
 
   return UKM_XML_TYPE.PrettyPrint(config)