[Extensions] Simplify Extension Keybinding API Tests

chrome/test/data/extensions/api_test/keybinding/conflicting/background.js

 // Copyright (c) 2012 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.
 
 // Called when the user clicks on the browser action.
 chrome.browserAction.onClicked.addListener(function(windowId) {
-  chrome.tabs.executeScript(null, { code: "document.body.bgColor='green'" });
+  chrome.test.sendMessage('conflicting browser action');
 });
 
 // Called when the user activates the command.
 chrome.commands.onCommand.addListener(function(command) {
-  chrome.tabs.executeScript(null, { code: "document.body.bgColor='yellow'" });
+  chrome.test.sendMessage('conflicting command');
 });
 
 chrome.test.notifyPass();