Add UMA metric to track the time saved on making events passive during fling

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 73786a1b2217519a507266e08319f93926487835..68def38614dfbdf82577a7338c36d50c89e6dc5d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -12315,6 +12315,24 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Event.Touch.TouchStartDuringFlingLatency" units="microseconds">

[Ilya Sherman, 2016/05/17 01:27:29]

nit: Perhaps "TouchStartLatencyDuringFling", and a similar change below?

+  <owner>tdresser@chromium.org</owner>
+  <summary>
+    Time between when a touch start event was generated and the event was
+    processed when there was an active fling animation. This histogram tracks
+    the benefit of forcing passive event listeners during fling.
+  </summary>
+</histogram>
+
+<histogram name="Event.Touch.TouchStartOutsideFlingLatency"
+    units="microseconds">
+  <owner>tdresser@chromium.org</owner>
+  <summary>
+    Time between when a touch start event was generated and the event was
+    processed when there was no active fling animation.

[Ilya Sherman, 2016/05/17 01:27:29]

nit: "was processed when" -> "was processed. Recorded only when" (and a similar
change above)

+  </summary>
+</histogram>
+
 <histogram name="Event.TouchDuration" units="ms">
   <owner>kuscher@google.com</owner>
   <owner>rbyers@chromium.org</owner>