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 104 matching lines @@
         "40063F1CF7B68BA847A26FA6620DDF156171D23C",
         "A6FD8E15353CF1F5C3D0A7B20E1D10AEA4DD3E6A",
         "57AC4D9E6BD8A2D0A70056B5FAC2378CAA588912",
         "02037314DA4D913640DCF0E296A7D01F4FD793EC",
         "B6EC0809BC63E10B431C5E4AA3645232CA86B2A5",
         "48CA541313139786F056DBCB504A1025CFF5D2E3",
         "05106136AE7F08A3C181D4648E5438350B1D2B4F"
       ]
     }
   ],
+  "app.window.alpha": [
+    {
+      "channel": "dev",
+      "extension_types": ["platform_app"]
+    },
+    {
+      "channel": "stable",
+      "extension_types": ["platform_app"],
+      "whitelist": [
+        "0F42756099D914A026DADFA182871C015735DD95",  // http://crbug.com/323773
+        "2D22CDB6583FD0A13758AEBE8B15E45208B4E9A7",
+        "E7E2461CE072DF036CF9592740196159E2D7C089",  // http://crbug.com/356200
+        "A74A4D44C7CFCD8844830E6140C8D763E12DD8F3",
+        "312745D9BF916161191143F6490085EEA0434997",
+        "53041A2FA309EECED01FFC751E7399186E860B2C"
+      ]
+    }
+  ],
   "audio": [
     {
       "channel": "dev",
       "extension_types": ["platform_app"]
     },
     {
       "channel": "stable",
       "extension_types": ["platform_app"],
       "whitelist": [
         "8C3741E3AF0B93B6E8E0DDD499BB0B74839EA578",
@@ skipping 997 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
     ]
   }]
 }