Add ScrollBegin.Touch/Wheel Event to UKM

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 251 matching lines @@
   </summary>
   <metric name="TimeToScrollUpdateSwapBegin">
     <summary>
       The time in microseconds between the initial creation of a touch event and
       the start of the frame swap on the GPU service. If no swap was induced by
       the event, no recording is made.
     </summary>
   </metric>
 </event>
 
+<event name="Event.ScrollBegin.Touch">
+  <owner>nzolghadr@chromium.org</owner>
+  <summary>
+    Metrics related to first scroll action caused by the generated ScrollUpdate
+    gesture event.
+  </summary>
+  <metric name="TimeToScrollUpdateSwapBegin">
+    <summary>
+      The time in microseconds between initial creation of a touch event and the
+      start of the frame swap on the GPU service caused by the generated
+      ScrollUpdate gesture event if that ScrollUpdate is the first such event in
+      a given scroll gesture event sequence. If no swap was induced by the
+      event, no recording is made.
+    </summary>
+  </metric>
+</event>
+
+<event name="Event.ScrollBegin.Wheel">
+  <owner>nzolghadr@chromium.org</owner>
+  <summary>
+    Metrics related to first scroll action caused by the generated ScrollUpdate
+    gesture event.
+  </summary>
+  <metric name="TimeToScrollUpdateSwapBegin">
+    <summary>
+      The time in microseconds between the initial creation of a wheel event and
+      the start of the frame swap on the GPU service caused by the generated
+      ScrollUpdate gesture event if that ScrollUpdate is the first such event in
+      a given scroll gesture event sequence. If no swap was induced by the
+      event, no recording is made.
+    </summary>
+  </metric>
+</event>
+
 <event name="Media.WatchTime">
   <owner>dalecurtis@chromium.org</owner>
   <summary>
     Watch time is defined as the amount of elapsed media time for audio+video
     media aggregated per player instance. A minimum of 7 seconds of unmuted,
     foreground media must be watched to start watch time monitoring. Watch time
     is checked on a regular basis and reported upon one of the stop events
     mentioned below or at player destruction if none occur prior.
 
     Any one of paused, hidden, or muted is sufficient to stop watch time metric
@@ skipping 161 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>