Log to the console when a dialog is suppressed or would be.

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"
 #include "chrome/browser/engagement/site_engagement_service.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/browser_list.h"
 #include "chrome/browser/ui/tab_modal_confirm_dialog.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
 #include "chrome/common/chrome_features.h"
 #include "components/app_modal/javascript_dialog_manager.h"
+#include "content/public/browser/render_frame_host.h"
 
 DEFINE_WEB_CONTENTS_USER_DATA_KEY(JavaScriptDialogTabHelper);
 
 namespace {
 
 bool IsEnabled() {
   return base::FeatureList::IsEnabled(features::kAutoDismissingDialogs);
 }
 
 app_modal::JavaScriptDialogManager* AppModalDialogManager() {
@@ skipping 50 matching lines @@
   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;
+      alerting_web_contents->GetMainFrame()->AddMessageToConsole(
+          content::CONSOLE_MESSAGE_LEVEL_WARNING,
+          "A window.prompt() dialog generated by this page was suppressed "
+          "because this page is not the active tab of the front window. "
+          "Please make sure your dialogs are triggered by user interactions "
+          "to avoid this situation. "
+          "https://www.chromestatus.com/feature/5637107137642496");
       return;
     }
 
     if (dialog_) {
       // There's already a dialog up; clear it out.
       CloseDialog(false, false, base::string16());
     }
 
     base::string16 title =
         AppModalDialogManager()->GetTitle(alerting_web_contents, origin_url);
@@ skipping 109 matching lines @@
   DCHECK(dialog_);
 
   dialog_->CloseDialogWithoutCallback();
   if (!suppress_callback)
     dialog_callback_.Run(success, user_input);
 
   dialog_.reset();
   dialog_callback_.Reset();
   BrowserList::RemoveObserver(this);
 }