webstorePrivate: Support an |authuser| property in beginInstallWithManifest3.

chrome/common/extensions/api/webstore_private.json

 // 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.
 
 [
   {
     "namespace":"webstorePrivate",
     "description": "none",
     "functions": [
       {
@@ skipping 93 matching lines @@
               },
               "appInstallBubble": {
                 "type": "boolean",
                 "optional": true,
                 "description": "A flag to change the UI we show when an app is installed - a value of true means to show a bubble pointing at the new tab button (instead of the default behavior of opening the new tab page and animating the app icon)."
               },
               "enableLauncher": {
                 "type": "boolean",
                 "optional": true,
                 "description": "A flag to cause the app launcher to be installed before installing the extension, if it isn't installed already."
+              },
+              "authuser": {
+                "type": "string",
+                "optional": true,
+                "description": "The authuser index to be included with CRX download requests in multi-login sessions."
               }
             },
             "additionalProperties": { "type": "any" }
           },
           {
             "name": "callback",
             "type": "function",
             "description": "Called when the user has either accepted/rejected the dialog, or some error occurred (such as invalid manifest or icon image data).",
             "optional": true,
             "parameters": [
@@ skipping 143 matching lines @@
             "type": "function",
             "description": "Called if the user is already signed in or if the sign-in method was invoked incorrectly (e.g. with an invalid continue URL), sign-in is unavailable (e.g. in Incognito), or the API was not called in response to a user gesture. Upon such a failure, chrome.runtime.lastError will be set to 'invalid_continue_url', 'signin_is_disallowed', 'signin_failed', 'merge_session_failed', or 'user_getsure_required'.",
             "optional": true,
             "parameters": []
           }
         ]
       }
     ]
   }
 ]