Add browser_test for extension app API with missing schema

extensions/common/api/_permission_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 permissions for 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.
 
 // To add a new whitelisted ID, SHA-1 it and force it to uppercase. In Bash:
 //
 // $ echo -n "aaaabbbbccccddddeeeeffffgggghhhh" | \
 // sha1sum | tr '[:lower:]' '[:upper:]'
 // 9A0417016F345C934A1A88F55CA17C05014EEEBA  -
 //
 // Google employees: please update http://go/chrome-api-whitelist to map
 // hashes back to ids.
 
+// If you add a new platform_app permission please update the "stubs_app" test:
+// chrome/test/data/extensions/api_test/stubs_app/manifest.json
+
 {
   "dns": [
     {
       "channel": "dev",
       "extension_types": ["extension", "platform_app"]
     },
     {
       "channel": "stable",
       "extension_types": ["extension", "platform_app"],
       "whitelist": [
@@ skipping 46 matching lines @@
     {
       "channel": "stable",
       "extension_types": ["extension"],
       "whitelist": [
         "496B6890097EB6E19809ADEADD095A8721FBB2E0",  // FIDO U2F APIs
         "E24F1786D842E91E74C27929B0B3715A4689A473"   // CryptoToken
       ]
     }
   ]
 }