Change the platform app manifest structure.

chrome/common/extensions/api/_manifest_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.
 
 {
   "app": {
     "channel": "stable",
     "extension_types": ["packaged_app", "hosted_app", "platform_app"]
   },
+  "app.background": {
+    "channel": "stable",
+    "extension_types": ["platform_app"],
+    "min_manifest_version": 2
+  },
   "app.launch": {
     "channel": "stable",
     "extension_types": ["packaged_app", "hosted_app"]
   },
   "app.isolation": {
     "channel": "stable",
     // Platform apps always have isolated storage, thus they cannot specify it
     // via the manifest.
     "extension_types": ["packaged_app", "hosted_app"]
   },
   "background": {
     "channel": "stable",
     "extension_types": [
-      "extension", "packaged_app", "hosted_app", "platform_app"
+      // Platform apps specify their background page via app.background.
+      "extension", "packaged_app", "hosted_app"
     ]
   },
   "background.persistent": {
     "channel": "dev",
     "extension_types": [
       "extension", "packaged_app"
     ],
     "min_manifest_version": 2
   },
   "background_page": {
     "channel": "stable",
     "extension_types": [
-      "extension", "packaged_app", "hosted_app", "platform_app"
+      "extension", "packaged_app", "hosted_app"
     ],
     "max_manifest_version": 1
   },
   "browser_action": {
     "channel": "stable",
     "extension_types": ["extension"]
   },
   "chrome_url_overrides": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"]
@@ skipping 122 matching lines @@
     "channel": "stable",
     "extension_types": ["extension"],
     "max_manifest_version": 1
   },
   "permissions": {
     "channel": "stable",
     "extension_types": [
       "extension", "packaged_app", "hosted_app", "platform_app"
     ]
   },
-  "platform_app": {
-    "channel": "stable",
-    "extension_types": ["platform_app"],
-    "min_manifest_version": 2
-  },
   "plugins": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app", "hosted_app"]
   },
   "requirements": {
     "channel": "stable",
     "extension_types": [
       "extension", "packaged_app", "hosted_app", "platform_app"
     ]
   },
@@ skipping 24 matching lines @@
     "channel": "stable",
     "extension_types": "all"
   },
   "web_accessible_resources": {
     "channel": "stable",
     "extension_types": [
       "extension", "packaged_app", "hosted_app"
     ]
   }
 }