For a "cancel" of an alert dialog via HandleJavaScriptDialog, click the only button, "OK".

components/app_modal/javascript_dialog_manager.cc

Index: components/app_modal/javascript_dialog_manager.cc
diff --git a/components/app_modal/javascript_dialog_manager.cc b/components/app_modal/javascript_dialog_manager.cc
index cec07c96d5fe325eeac4778f2b8d85f5eb95bbbc..c5a368384b597f56c031f3c1ad1c6b89f659e268 100644
--- a/components/app_modal/javascript_dialog_manager.cc
+++ b/components/app_modal/javascript_dialog_manager.cc
@@ -248,8 +248,17 @@ bool JavaScriptDialogManager::HandleJavaScriptDialog(
       dialog_queue->active_dialog()->web_contents() != web_contents) {
     return false;
   }
+
   JavaScriptAppModalDialog* dialog = static_cast<JavaScriptAppModalDialog*>(
       dialog_queue->active_dialog());
+
+  if (dialog->javascript_dialog_type() ==
+      content::JavaScriptDialogType::JAVASCRIPT_DIALOG_TYPE_ALERT) {
+    // Alert dialogs only have one button: OK. Any "handling" of this dialog has
+    // to be a click on the OK button.
+    accept = true;
+  }
+
   if (accept) {
     if (prompt_override)
       dialog->SetOverridePromptText(*prompt_override);