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() {
+    chrome.printerProvider.onGetPrintersRequested.addListener(
+        function(callback) {
+          chrome.test.assertFalse(!!chrome.printerProviderInternal);
+          chrome.test.assertTrue(!!callback);
+
+          chrome.usb.getDevices({}, function(devices) {
+            chrome.test.assertNoLastError();
+            var printers = [];
+            for (var device of devices) {
+              printers.push({
+                'id': 'enumerated-' + device.device,
+                'name': 'Test Printer',
+                'description': 'This printer is a USB device.',
+              });
+            }
+            callback(printers);
+            chrome.test.succeed();
+          });
+        });
+
+    chrome.printerProvider.onUsbAccessGranted.addListener(
+        function(device, callback) {
+          chrome.test.assertFalse(!!chrome.printerProviderInternal);
+          chrome.test.assertTrue(!!callback);
+
+          callback({
+            'id': 'granted-' + device.device,

[tbarzic, 2015/05/27 00:11:03]

generally, destinations for a printer returned by onGetPrintersRequested and
onUsbAccessGranted should have the same id, so I'd use the same id here also.

[Reilly Grant (use Gerrit), 2015/05/28 21:04:10]

Done.

+            '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.sendMessage('ready');
+  }]);
+});