Add TimeToScrollUpdateSwapBegin2 to UKM

tools/metrics/ukm/ukm.xml

Index: tools/metrics/ukm/ukm.xml
diff --git a/tools/metrics/ukm/ukm.xml b/tools/metrics/ukm/ukm.xml
index ab75b3a1ed0b53dd279b96f8a32f41e1e48e44a4..4f07774a2dd13fed5a4b9db58ac531e47dc41eea 100644
--- a/tools/metrics/ukm/ukm.xml
+++ b/tools/metrics/ukm/ukm.xml
@@ -234,6 +234,20 @@ be describing additional metrics about the same event.
   </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>