Export a private webstore API to call into the new inline sign-in flow.

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 234 matching lines @@
         "parameters": [
           {
             "name": "callback",
             "type": "function",
             "optional": false,
             "parameters": [
               { "name": "is_incognito", "type": "boolean" }
             ]
           }
         ]
+      },
+      {
+        "name": "signIn",
+        "description": "Signs the user into Chrome.",
+        "parameters": [
+          {
+            "name": "continue_url",
+            "type": "string",
+            "description": "The continue URL to navigate to once sign-in completes."
+          },
+          {
+            "name": "callback",
+            "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": []
+          }
+        ]
       }
     ]
   }
 ]