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

chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.h"
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/feature_list.h"
 #include "base/metrics/histogram_macros.h"
@@ skipping 58 matching lines @@
     UMA_HISTOGRAM_COUNTS("JSDialogs.CharacterCount.EngagementLessThanOne",
                          message_length);
   } else if (engagement_score < 5) {
     UMA_HISTOGRAM_COUNTS("JSDialogs.CharacterCount.EngagementOneToFive",
                          message_length);
   } else {
     UMA_HISTOGRAM_COUNTS("JSDialogs.CharacterCount.EngagementHigher",
                          message_length);
   }
 
+  content::WebContents* parent_web_contents =
+      WebContentsObserver::web_contents();
+  bool foremost = IsWebContentsForemost(parent_web_contents);
+  switch (message_type) {
+    case content::JAVASCRIPT_MESSAGE_TYPE_ALERT:
+      UMA_HISTOGRAM_BOOLEAN("JSDialogs.IsForemost.Alert", foremost);
+      break;
+    case content::JAVASCRIPT_MESSAGE_TYPE_CONFIRM:
+      UMA_HISTOGRAM_BOOLEAN("JSDialogs.IsForemost.Confirm", foremost);
+      break;
+    case content::JAVASCRIPT_MESSAGE_TYPE_PROMPT:
+      UMA_HISTOGRAM_BOOLEAN("JSDialogs.IsForemost.Prompt", foremost);
+      break;
+  }
+
   if (IsEnabled()) {
-    content::WebContents* parent_web_contents =
-        WebContentsObserver::web_contents();
-
     if (!IsWebContentsForemost(parent_web_contents) &&
         message_type == content::JAVASCRIPT_MESSAGE_TYPE_PROMPT) {
       // Don't allow "prompt" dialogs to steal the user's focus. TODO(avi):
       // Eventually, for subsequent phases of http://bit.ly/project-oldspice,
       // turn off focus stealing for other dialog types.
       *did_suppress_message = true;
       return;
     }
 
     if (dialog_) {
@@ skipping 115 matching lines @@
   DCHECK(dialog_);
 
   dialog_->CloseDialogWithoutCallback();
   if (!suppress_callback)
     dialog_callback_.Run(success, user_input);
 
   dialog_.reset();
   dialog_callback_.Reset();
   BrowserList::RemoveObserver(this);
 }