Add TimeToScrollUpdateSwapBegin2 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 216 matching lines @@
     </summary>
   </metric>
   <metric name="WAS_SCREEN_BOTTOM">
     <summary>
       The location of the tap relative to the bottom of the screen, expressed as
       an integer with 0 meaning not at the bottom and 1 meaning at the bottom.
     </summary>
   </metric>
 </event>
 
+<event name="Event.Latency.ScrollUpdate.Touch.TimeToScrollUpdateSwapBegin2">
+  <owner>nzolghadr@chromium.org</owner>
+  <summary>
+    The time between the 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 no swap was induced by the event, no recording is made.
+  </summary>
+  <metric name="Latency">
+    <summary>
+      Latency in microseconds representing Touch.TimeToScrollUpdateSwapBegin.

[tdresser, 2017/05/30 16:52:07]

This summary is a bit confusing. Do we need a metric specific summary here? Will
this event ever have multiple entries?

Long term, it would make sense to have an event which shared the top level
metric as well as the breakdown metrics. Perhaps we should name things such that
this naturally extends to include the breakdowns?

[rkaplow, 2017/05/30 20:03:32]

I agree - does it make more sense for the Event to be a more generic Scroll
Touch? Is there other metrics related to it that may want to eb added and
associated with it?

[Navid Zolghadr, 2017/05/31 20:37:15]

I did some renaming here. Let me know if this looks better now.

+    </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 86 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>