<webview>: First stab at implementing media permission request for guests.

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.
 
 {
   "activeTab": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"],
     "min_manifest_version": 2
   },
@@ skipping 361 matching lines @@
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"]
   },
   "webRequestBlocking": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"]
   },
   "webview": {
     "channel": "dev",
     "extension_types": ["platform_app"]
+  },
+  "webview.permissionAPI": {
+    "channel": "dev",
+    "extension_types": ["platform_app"]
   }
 }