Add onGetUsbPrinterInfoRequested event to printerProvider API.

extensions/test/data/api_test/printer_provider/usb_printers/test.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.
+
+chrome.test.sendMessage('loaded', function(test) {
+  chrome.test.runTests([function printTest() {
+    if (test == 'NO_LISTENER') {
+      chrome.test.sendMessage('ready');
+      chrome.test.succeed();
+      return;
+    }
+
+    chrome.printerProvider.onGetUsbPrinterInfoRequested.addListener(
+        function(device, callback) {
+          chrome.test.assertFalse(!!chrome.printerProviderInternal);
+          chrome.test.assertTrue(!!callback);
+
+          if (test == 'EMPTY_RESPONSE') {
+            callback();
+          } else {
+            callback({
+              'id': 'usbDevice-' + device.device,
+              'name': 'Test Printer',
+              'description': 'This printer is a USB device.',
+            });
+          }
+
+          chrome.test.assertThrows(
+              callback,
+              [],
+              'Event callback must not be called more than once.');
+
+          chrome.test.succeed();
+        });
+
+    chrome.test.sendMessage('ready');
+  }]);
+});