UMA stats for stylus usage

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 378701f806a60e4b7402dc4c195bf67970d6504c..34afc7f1084ae1ca87c51401366d1f3f42c2f3d8 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -12789,6 +12789,12 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Event.DownEventCount.PerDestination"
+    enum="DownEventDestination">
+  <owner>xiaoyinh@chromium.org</owner>
+  <summary>Counts the number of down events received per destination.</summary>

[Mark P, 2016/09/16 20:27:02]

nit: omit "Counts"

[Mark P, 2016/09/16 20:27:02]

Add clarification that this only emitted for events that are handled / have an
effect.
(As an example to test the clarify of what you add, consider whether it answer
the question: What about tapping on a disabled button?)

[xiaoyinh(OOO Sep 11-29), 2016/09/16 22:34:25]

Done.

[xiaoyinh(OOO Sep 11-29), 2016/09/16 22:34:25]

Actually it will also count events that has no effect. As long as the event
target belongs to the category we are interested in. Tapping on a disabled
button inside the browser frame will be treated as down events on browser
window. 

I've added this information in the summary. Hope it can make things more clear.

Thanks

+</histogram>
+
 <histogram name="Event.DownEventCount.PerFormFactor" enum="DownEventFormFactor">
   <owner>xiaoyinh@chromium.org</owner>
   <summary>
@@ -75543,6 +75549,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="2" label="GET_NON_CACHABLE"/>
 </enum>
 
+<enum name="DownEventDestination" type="int">
+  <int value="0" label="Others, everything except browser and apps"/>
+  <int value="1" label="Inside the browser frame"/>
+  <int value="2" label="Regular chrome app, except default note-taking app"/>
+  <int value="3" label="ARC++ app(andriod app on Chrome)"/>

[Mark P, 2016/09/16 20:27:02]

nit: space before (

[xiaoyinh(OOO Sep 11-29), 2016/09/16 22:34:25]

Done.

+  <int value="4" label="Default note-taking app"/>
+</enum>
+
 <enum name="DownEventFormFactor" type="int">
   <int value="0" label="Clamshell"/>
   <int value="1" label="Touchview"/>