Add chrome.usbPrivate API for use by USB device WEBUI.

extensions/common/api/_api_features.json

 // Copyright 2014 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.
 
 // This features file defines extension APIs implemented under src/extensions.
 // See extensions/common/features/* to understand this file, in particular
 // feature.h, simple_feature.h, and base_feature_provider.h.
 //
 // Note that specifying "web_page", "blessed_web_page", or "all" as a context
 // type will require manually updating chrome/renderer/resources/dispatcher.cc.
@@ skipping 137 matching lines @@
     "contexts": ["blessed_extension"]
   },
   "types.private": {
     "channel": "dev",
     "extension_types": ["extension"],
     "location": "component"
   },
   "usb": {
     "dependencies": ["permission:usb"],
     "contexts": ["blessed_extension"]
+  },
+  "usbPrivate": {
+    "channel": "dev",
+    "contexts": ["webui"]
   }
 }