Add browser tests for USB device add/remove events.

extensions/test/data/api_test/usb/add_event/background.js

-// Copyright 2014 The Chromium Authors. All rights reserved.
+// Copyright 2015 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.
 
-chrome.hid.onDeviceAdded.addListener(function (device) {
+chrome.usb.onDeviceAdded.addListener(function (device) {

[Yoyo Zhou, 2015/01/21 06:57:43]

nit: while you're here, delete the space after function.

[Reilly Grant (use Gerrit), 2015/01/22 00:45:01]

I swear some linter complained about me not putting spaces after function but
Google's style guide agrees with you.

   if (device.vendorId == 6353 && device.productId == 22768) {
     chrome.test.sendMessage("success");
   } else {
     console.error("Got unexpected device: vid:" + device.vendorId +
                   " pid:" + device.productId);
     chrome.test.sendMessage("failure");
   }
 });
 chrome.test.sendMessage("loaded");