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

chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc

Index: chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
diff --git a/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc b/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
index dd52e788c6c634380fed2c5ca9a9966a4076e6e1..1dfd9f48b1d8be5708f131de3ce0e0f8f5b145d4 100644
--- a/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
+++ b/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
@@ -16,6 +16,7 @@
 #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);
 
@@ -86,6 +87,13 @@ void JavaScriptDialogTabHelper::RunJavaScriptDialog(
       // 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;
     }