Change the platform app manifest structure.

chrome/test/data/extensions/manifest_tests/sandboxed_pages_invalid_5.json

 {
   "name": "test",
   "version": "0.1",
   "manifest_version": 2,
-  "platform_app": true,
-  "background": {
-    "scripts": ["background.js"]
+  "app": {
+    "background": {
+      "scripts": ["background.js"]
+    }
   },
   "sandbox": {
     "pages": ["test"],
     // Sandbox directive that allows top-level navigation in a platform app.
     "content_security_policy": "sandbox allow-top-navigation"
   }
 }