Take notification API out of experimental.

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 253 matching lines @@
   "networkingPrivate": {
     "channel": "dev",
     "extension_types": ["extension", "packaged_app"],
     "whitelist": [
       "pkedcjkdefgpdelpbcmbmeomcjbeemfm",  // CV
       "fmfcbgogabcbclcofgocippekhfcmgfj",  // CV Staging
       "hfaagokkkhdbgiakmmlclaapfelnkoah"   // CV Canary
     ]
   },
   "notifications": {
+    // Note that the chrome.notifications API is currently restricted
+    // to dev channel, extension/platform_app. This isn't done through
+    // this file, but rather on a method-by-method basis.
     "channel": "stable",
     "extension_types": [
       "extension", "packaged_app", "hosted_app", "platform_app"
     ]
   },
   "echoPrivate": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"],
     "location": "component"
   },
@@ skipping 164 matching lines @@
   },
   "webRequestBlocking": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"]
   },
   "webview": {
     "channel": "stable",
     "extension_types": ["platform_app"]
   }
 }