Add a UsbService::Observer function for cleanup actions.

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

+// 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.
+
+var device_from_user = undefined;
+
+chrome.test.runWithUserGesture(function() {
+  chrome.usb.getDevices({}, function(devices) {
+    chrome.test.assertEq(0, devices.length);
+    chrome.usb.getUserSelectedDevices({ multiple: false }, function(devices) {
+      chrome.test.assertEq(1, devices.length);
+      device_from_user = devices[0];
+      chrome.usb.openDevice(device_from_user, function(connection) {
+        chrome.usb.closeDevice(connection);
+        chrome.test.sendMessage("opened_device");
+      });
+    });
+  });
+});
+
+chrome.usb.onDeviceRemoved.addListener(function(device) {
+  if (device.device == device_from_user.device) {
+    chrome.test.sendMessage("success");
+  } else {
+    chrome.test.sendMessage("failure");
+  }
+});