Add UMA metric for tracking listeners for blocking touch while fling is happening

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 8d4ef5fe94a93b1eca31786692d416c13dc5487b..91364ab2589b2d4863d272b35fcf3738c5c53348 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -12164,6 +12164,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Event.Touch.TouchStartsCanceledDuringFling"
+    enum="BooleanHandledTouchesOnPageload">
+  <owner>tdresser@chromium.org</owner>
+  <summary>
+    Count the handled touch start listeners while there is an active fling
+    animation.
+  </summary>
+</histogram>
+
 <histogram name="Event.TouchDuration" units="ms">
   <owner>kuscher@google.com</owner>
   <owner>rbyers@chromium.org</owner>
@@ -62538,6 +62547,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Has predictions"/>
 </enum>
 
+<enum name="BooleanHandledTouchesOnPageload" type="int">

[lanwei, 2016/04/27 19:01:57]

I will rename this one to be more general.

+  <int value="0" label="Unhandled touch events"/>
+  <int value="1" label="Handled touch events"/>
+</enum>
+
 <enum name="BooleanHandshakeConfirmed" type="int">
   <int value="0" label="Handshake not confirmed"/>
   <int value="1" label="Handshake confirmed"/>