Mark PageLoad UKM as singular.

tools/metrics/ukm/ukm.xml

 <!--
 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.
 -->
 
 <!--
 This file is used to generate a comprehensive list of Chrome UKM metrics
 along with a detailed description for each metric.
+
+Events may be marked with the attribute singular="True" to indicate that
+the event will only occur once per source, and multiple entries will just
+be describing additional metrics about the same event.
 -->
 
 <ukm-configuration>
 
 <event name="Autofill.CardUploadDecision">
   <owner>sebsg@chromium.org</owner>
   <metric name="UploadDecision">
     <summary>
       Whether the upload was proposed to the user or the reasons why it was not.
       The value is a bitmask of |CardUploadDecisionMetric|.
@@ skipping 128 matching lines @@
     </summary>
   </metric>
   <metric name="ServerType">
     <summary>
       Field's |ServerFieldType| returned by server. See
       |AutofillField.server_type()|.
     </summary>
   </metric>
 </event>
 
-<event name="PageLoad">
+<event name="PageLoad" singular="True">
   <owner>bmcquade@chromium.org</owner>
   <summary>
     Recorded when Pageload observer events fire.
   </summary>
   <metric name="DocumentTiming.NavigationToDOMContentLoadedEventFired">
     <summary>
       Time from navigation to dom content loaded in ms.
     </summary>
   </metric>
   <metric name="DocumentTiming.NavigationToLoadEventFired"/>
@@ skipping 46 matching lines @@
   <metric name="DeclineCount"/>
   <metric name="EventType"/>
   <metric name="IgnoreCount"/>
   <metric name="RankerResponse"/>
   <metric name="RankerVersion"/>
   <metric name="SourceLanguage"/>
   <metric name="TargetLanguage"/>
 </event>
 
 </ukm-configuration>