Add UMA metrics to determine the usefulness of passive event listeners.

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 b4f77cb3bd008baa95658385455fdfdc7b0609ab..588257f6b949a59276d468c8516e4b5e2cca5dfb 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -10710,6 +10710,23 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Event.PassiveListeners" enum="EventResultType">

[tdresser, 2015/11/23 21:14:34]

I'm not sure I've seen this pattern before, where one histogram name is the
prefix of a different histogram name. It might make sense to switch this to
Event.PassiveListeners.Result. The histograms.xml reviewer will probably know
whether that would be preferable.

+  <owner>dtapuska@chromium.org</owner>
+  <summary>
+    The result of handling of MouseWheel, TouchStart, TouchMove, TouchEnd events
+    in the renderer.
+  </summary>
+</histogram>
+
+<histogram name="Event.PassiveListeners.Latency" units="microseconds">
+  <owner>dtapuska@chromium.org</owner>
+  <summary>
+    Time between when a cancelable event was generated and the event processed
+    yet no action was executed for the event. This histogram tracks the

[tdresser, 2015/11/23 21:14:34]

"yet" -> "for cases when"
"no action was executed" -> "preventDefault wasn't called"?

+    potential benefit of using passive events listeners.
+  </summary>
+</histogram>
+
 <histogram name="Event.SingleTapType" enum="TapDelayType">
   <owner>rbyers@chromium.org</owner>
   <summary>
@@ -59459,6 +59476,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="128" label="Request type matched."/>
 </enum>
 
+<enum name="EventResultType" type="int">
+  <int value="0" label="Passive"/>
+  <int value="1" label="Uncancelable"/>
+  <int value="2" label="Cancelable and not canceled"/>
+  <int value="3" label="Cancelable and canceled"/>
+</enum>
+
 <enum name="EVWhitelistStatus" type="int">
   <int value="0" label="Not present"/>
   <int value="1" label="Invalid"/>