Add a UMA counter for tap disambiguation result.

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 303ecd95778148fe122524c94f37dde5b5747e04..b8b8a765f51895283d9d12c0f1d972146e2f43ec 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -72817,6 +72817,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <owner>pthammaiah@google.com</owner>
 </histogram>
 
+<histogram name="Touchscreen.TapDisambiguation" enum="TapDisambiguation">
+  <owner>aelias@chromium.org</owner>
+  <summary>
+    How the user interacted with the tap disambiguation feature on Android.
+  </summary>
+</histogram>
+
 <histogram name="Touchscreen.TouchEventsEnabled"
     enum="TouchEventFeatureDetectionState">
   <owner>tdresser@chromium.org</owner>
@@ -112305,6 +112312,13 @@ from previous Chrome versions.
   <int value="1" label="Undelayed Tap"/>
 </enum>
 
+<enum name="TapDisambiguation" type="int">
+  <int value="0" label="TappedInside"/>

[Ilya Sherman, 2017/03/24 22:46:35]

Optional nit: It's fine to write this as "Tapped inside" if you'd like.

+  <int value="1" label="TappedOutside"/>
+  <int value="2" label="BackButton"/>
+  <int value="3" label="Other"/>
+</enum>
+
 <enum name="TcpSocketStatus" type="int">
   <int value="0" label="Unknown"/>
   <int value="1" label="Fast Connection Return"/>