webstorePrivate: Support an |authuser| property in beginInstallWithManifest3.

chrome/test/data/extensions/api_test/webstore_private/begin_install.html

 <!--
  * Copyright (c) 2012 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.
 -->
 <script src="common.js"></script>
 <script>
 
 runTests([
   function beginInstallWithAppBubble() {
     var manifest = getManifest("app/manifest.json");
     chrome.webstorePrivate.beginInstallWithManifest3(
-        {id: appId, manifest: manifest, appInstallBubble: true},
+        {id: appId, manifest: manifest, appInstallBubble: true, authuser: "2"},
         callbackPass(function(result) {
       assertEq("", result);
     }));
   },
 
   function beginInstallWithNoAppBubble() {
     var manifest = getManifest();
     chrome.webstorePrivate.beginInstallWithManifest3(
         {id: extensionId, manifest: manifest},
         callbackPass(function(result) {
       assertEq("", result);
     }));
   },
 ]);
 
 </script>