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

components/app_modal/javascript_dialog_manager.cc

 // Copyright 2014 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 "components/app_modal/javascript_dialog_manager.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 230 matching lines @@
 bool JavaScriptDialogManager::HandleJavaScriptDialog(
     content::WebContents* web_contents,
     bool accept,
     const base::string16* prompt_override) {
   AppModalDialogQueue* dialog_queue = AppModalDialogQueue::GetInstance();
   if (!dialog_queue->HasActiveDialog() ||
       !dialog_queue->active_dialog()->IsJavaScriptModalDialog() ||
       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);
     dialog->native_dialog()->AcceptAppModalDialog();
   } else {
     dialog->native_dialog()->CancelAppModalDialog();
   }
   return true;
 }
 
@@ skipping 44 matching lines @@
   // lazy background page after the dialog closes. (Dialogs are closed before
   // their WebContents is destroyed so |web_contents| is still valid here.)
   extensions_client_->OnDialogClosed(web_contents);
 
   last_close_time_ = base::TimeTicks::Now();
 
   callback.Run(success, user_input);
 }
 
 }  // namespace app_modal