Add support for transparent background API

chrome/common/extensions/api/_permission_features.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.
 
 // This features file defines permissions for extension APIs implemented
 // under src/chrome.
 // 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:
@@ skipping 48 matching lines @@
     "extension_types": ["platform_app"]
   },
   "app.window.fullscreen.overrideEsc": {
     "channel": "stable",
     "extension_types": ["platform_app"]
   },
   "app.window.shape": {
     "channel": "dev",
     "extension_types": ["platform_app"]
   },
+  "app.window.alpha": {
+    "channel": "dev",
+    "extension_types": ["platform_app"]
+  },
   "audio": [
     {
       "channel": "dev",
       "extension_types": ["platform_app"]
     },
     {
       "channel": "stable",
       "extension_types": ["platform_app"],
       "whitelist": [
         "8C3741E3AF0B93B6E8E0DDD499BB0B74839EA578",
@@ skipping 888 matching lines @@
     // Only allowed for whitelisted extensions until all the caveats are
     // addressed. Tracked in crbug/285151.
     "channel": "stable",
     "extension_types": ["extension"],
     "location": "component",
     "whitelist": [
       "D519188F86D9ACCEE0412007B227D9936EB9676B"  // GAIA Component Extension
     ]
   }]
 }