Add UMA metrics to measure the dismissal cause of dialogs.

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 50c656f7c7a6c30490e717e4c25ee2631bfd9119..c48d0791184a3025cfd2c7642d56b39c83b1e0fb 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -22081,6 +22081,12 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="JSDialogs.DismissalCause"
+    enum="JavaScriptDialogDismissalCause">
+  <owner>avi@chromium.org</owner>
+  <summary>The cause of dismissal of JavaScript dialogs.</summary>
+</histogram>
+
 <histogram
     name="JSDialogs.FineTiming.TimeBetweenDialogClosedAndNextDialogCreated"
     units="ms">
@@ -90269,6 +90275,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="7" label="VideoCaptureStream"/>
 </enum>
 
+<enum name="JavaScriptDialogDismissalCause" type="int">
+  <int value="0" label="Tab closed">The tab owning the dialog was closed</int>
+  <int value="1" label="New dialog">
+    A new dialog was shown, closing the existing dialog
+  </int>
+  <int value="2" label="HandleDialog">HandleJavaScriptDialog was called</int>
+  <int value="3" label="CancelDialog">CancelDialogs was called</int>
+  <int value="4" label="Tab hidden">The tab owning the dialog was hidden</int>
+  <int value="5" label="Browser switched">
+    The browser owning the dialog was deactivated
+  </int>
+  <int value="6" label="Button click">
+    The user clicked on the OK or Cancel button
+  </int>
+</enum>
+
 <enum name="JumplisticonsfolderCategory" type="int">
   <int value="0" label="Del Succeed - Mov Succeed - Create Succeed"/>
   <int value="1" label="Del Fail - Mov Succeed - Create Succeed"/>
@@ -108411,6 +108433,7 @@ value.
   <suffix name="Alert"/>
   <suffix name="Confirm"/>
   <suffix name="Prompt"/>
+  <affected-histogram name="JSDialogs.DismissalCause"/>
   <affected-histogram name="JSDialogs.IsForemost"/>
   <affected-histogram name="JSDialogs.SiteEngagementOfDialogs"/>
 </histogram_suffixes>