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 3de48e1fb7388ea4fe0a227c7523a59bae1dcff3..2bd1e9e010855d8c8962cc361964f19bedc6ab23 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -12800,6 +12800,17 @@ 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>
+    The number of down events received per destination. Every down event that is
+    targeted to each destination will be counted, including those that don't
+    have an effect. For example: Tapping on a disabled button inside the browser
+    frame will be treated as down events on browser window.
+  </summary>
+</histogram>
+
 <histogram name="Event.DownEventCount.PerFormFactor" enum="DownEventFormFactor">
   <owner>xiaoyinh@chromium.org</owner>
   <summary>
@@ -75810,6 +75821,14 @@ 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 (android app on Chrome)"/>
+  <int value="4" label="Default note-taking app"/>
+</enum>
+
 <enum name="DownEventFormFactor" type="int">
   <int value="0" label="Clamshell"/>
   <int value="1" label="Touchview"/>