Add a test for requesting mount via a context menu in Files app.

ui/file_manager/integration_tests/testing_provider/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.
+
+'use strict';
+
+var dialogSettings = {};
+
+chrome.fileSystemProvider.onGetMetadataRequested.addListener(
+    function(options, onSuccess, onError) {
+      onSuccess({
+        isDirectory: true,
+        name: '',
+        size: 0,
+        modificationTime: new Date()
+      });
+    });
+
+chrome.fileSystemProvider.onReadDirectoryRequested.addListener(
+    function(options, onSuccess, onError) {
+      onSuccess([], false /* hasMore */);
+    });
+
+chrome.fileSystemProvider.onMountRequested.addListener(
+    function(onSuccess, onError) {
+      chrome.fileSystemProvider.getAll(function(mounted) {
+        var index = mounted.length + 1;
+        chrome.fileSystemProvider.mount({
+          fileSystemId: 'test-fs-' + index,
+          displayName: 'Test (' + index + ')'
+        });
+      });
+    });
+
+chrome.fileSystemProvider.onUnmountRequested.addListener(
+    function(options, onSuccess, onError) {
+      chrome.fileSystemProvider.unmount(
+          {
+            fileSystemId: options.fileSystemId
+          },
+          function() {
+            if (chrome.runtime.lastError)
+              onError(chrome.runtime.lastError.message);
+            else
+              onSuccess();
+          });
+    });