Adding a destination platform histogram for UMA.

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 93ea14f12ed4a018cc65c2f10994b2c3a7290c46..76cc85d200edf437b1406af013c385d1ecc0d31c 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -1221,6 +1221,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Arc intent handler action taken by user.</summary>
 </histogram>
 
+<histogram name="Arc.IntentHandlerSelectedPlatform"
+    enum="ArcIntentHandlerSelectedPlatform">
+  <owner>elijahtaylor@google.com</owner>
+  <owner>mitsuji@google.com</owner>
+  <summary>
+    ARC intent handler selected platform. The selection may have happened due to
+    the user explicit selection or a previously stored preference.
+  </summary>
+</histogram>
+
 <histogram name="Arc.LowMemoryKiller.Count">
   <owner>elijahtaylor@google.com</owner>
   <owner>mitsuji@google.com</owner>
@@ -73462,6 +73472,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="4" label="Preferred activity found"/>
 </enum>
 
+<enum name="ArcIntentHandlerSelectedPlatform" type="int">
+  <summary>
+    Defines Arc intent handler platforms to continue the navigation.

[elijahtaylor1, 2016/11/03 22:22:00]

s/Arc/ARC

[djacobo_, 2016/11/03 23:13:55]

Done.

+  </summary>
+  <int value="0" label="ARC"/>
+  <int value="1" label="Chrome"/>
+</enum>
+
 <enum name="ArcOptInAction" type="int">
   <summary>Defines Arc OptIn actions</summary>
   <int value="0" label="Opted In"/>