Add UMA metrics to see the proportions of dialogs spawned by foremost pages.

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 5705fcf47993a258bed634cbb4585acfd061de96..a75250898029259c20ab61c8389baf51caf7b4b0 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -21977,6 +21977,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="JSDialogs.IsForemost" enum="BooleanForemost">
+  <owner>avi@chromium.org</owner>
+  <summary>
+    For dialogs, whether or not they were spawned by a tab that was foremost.
+  </summary>
+</histogram>
+
 <histogram name="JSDialogs.OnBeforeUnloadStayVsLeave" enum="StayVsLeave">
   <owner>avi@chromium.org</owner>
   <summary>
@@ -75590,6 +75597,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Force Disabled"/>
 </enum>
 
+<enum name="BooleanForemost" type="int">
+  <int value="0" label="Tab was not foremost"/>
+  <int value="1" label="Tab was foremost"/>
+</enum>
+
 <enum name="BooleanFormatChanged" type="int">
   <int value="0" label="Any Codec Initialized"/>
   <int value="1" label="Missing FORMAT_CHANGED message"/>
@@ -107580,12 +107592,10 @@ value.
 </histogram_suffixes>
 
 <histogram_suffixes name="JSDialogs.DialogType" separator=".">
-  <obsolete>
-    Deprecated 2016-10.
-  </obsolete>
   <suffix name="Alert"/>
   <suffix name="Confirm"/>
   <suffix name="Prompt"/>
+  <affected-histogram name="JSDialogs.IsForemost"/>
   <affected-histogram name="JSDialogs.SiteEngagementOfDialogs"/>
 </histogram_suffixes>