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..13579b2dfc2cb3deab32de0b4b7dcba17c20183f 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 +90275,16 @@ 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="The tab owning the dialog was closed"/>
+  <int value="1" label="A subsequent dialog was shown, closing the dialog"/>
+  <int value="2" label="HandleJavaScriptDialog was called"/>
+  <int value="3" label="CancelDialogs was called"/>
+  <int value="4" label="The tab owning the dialog was hidden"/>
+  <int value="5" label="The browser owning the dialog was deactivated"/>
+  <int value="6" label="The user clicked on the OK or Cancel button"/>

[Ilya Sherman, 2016/11/29 03:10:35]

Optional: These labels are pretty long, and might be hard to read in the
dashboard UI.  You might want to provide shorter labels, with longform hover
text, like so:

<int value="6" label="OK/Cancel button">The user clicked on the OK or Cancel
button</int>

[Avi (use Gerrit), 2016/11/29 16:10:59]

I didn't know that was possible! Done.

+</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 +108427,7 @@ 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>