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..074cc8148f22d0519b4e2105a96e6b72a4036732 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -12315,6 +12315,23 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Event.Touch.TouchStartDuringFlingLatency" units="microseconds">
+  <owner>tdresser@chromium.org</owner>
+  <summary>
+    Time between when a touch start event was generated and the event processed
+    while there is an active fling animation. This histogram tracks the benefit
+    of forcing passive event listeners during fling.
+  </summary>
+</histogram>
+
+<histogram name="Event.Touch.TouchStartNoFlingLatency" units="microseconds">
+  <owner>tdresser@chromium.org</owner>
+  <summary>
+    Time between when a touch start event was generated and the event processed
+    while there is no active fling animation.

[tdresser, 2016/05/12 12:11:32]

The tenses are a bit weird here.

"while there is" -> "when there was"?

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