Add a private API method to install a webstore app from Files.app

chrome/common/extensions/api/file_browser_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":"fileBrowserPrivate",
     "description": "none",
     "platforms": ["chromeos"],
     "types": [
@@ skipping 1114 matching lines @@
             "type": "function",
             "parameters": [
               {
                 "name": "shareUrl",
                 "type": "string",
                 "description": "Share Url for the sharing dialog."
               }
             ]
           }
         ]
+      },
+      {
+        "name": "installWebstoreItem",
+        "type": "function",
+        "description": "Requests to install a webstore item.",
+        "parameters": [
+          {
+            "name": "item_id",
+            "type": "string",
+            "description": "The id of the item to install."
+          },
+          {
+            "name": "callback",
+            "type": "function",
+            "parameters": []
+          }
+        ]
       }
     ],
     "events": [
       {
         "name": "onMountCompleted",
         "type": "function",
         "description": "Fired when mount event is detected.",
         "parameters": [
           {
             "$ref": "MountCompletedEvent",
@@ skipping 35 matching lines @@
       },
       {
         "name": "onDriveConnectionStatusChanged",
         "type": "function",
         "description": "Fired when the active network connection state changes. The network connection state can be retrieved via 'getDriveConnectionState'.",
         "parameters": []
       }
     ]
   }
 ]