Add Flash to UKM entry list

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.
 -->
@@ skipping 179 matching lines @@
   </metric>
   <metric name="Events">
     <summary>
       Bitfield representing the events that occurred during the Payment Request.
       Values defined in the Event enum of
       components/payments/core/journey_logger.h.
     </summary>
   </metric>
 </event>
 
+<event name="Plugins.FlashInstance">
+  <owner>tommycli@chromium.org</owner>
+  <summary>
+    Recorded when a Flash object instance is created.
+  </summary>
+</event>
+
 <event name="Translate">
   <owner>hamelphi@chromium.org</owner>
   <summary>
     Metrics related to a Translate event. These metrics are described in
     TranslateEventProto.
   </summary>
   <metric name="AcceptCount"/>
   <metric name="Country"/>
   <metric name="DeclineCount"/>
   <metric name="EventType"/>
   <metric name="IgnoreCount"/>
   <metric name="RankerResponse"/>
   <metric name="RankerVersion"/>
   <metric name="SourceLanguage"/>
   <metric name="TargetLanguage"/>
 </event>
 
 </ukm-configuration>