Move chrome.experimental.app.onLaunched event handler to chrome.app.runtime.onLaunched.

chrome/test/data/extensions/platform_apps/get_display_path/manifest.json

 {
   "name": "Platform App fileSystem.getDisplayPath test",
   "version": "1",
   "manifest_version": 2,
   "intents": {
     "http://webintents.org/view": {
       "type": [
         "text/*"
       ],
       "title": "Test editor"
     }
   },
-  "permissions": ["experimental", "fileSystem"],
+  "permissions": [
+    "fileSystem"
+  ],
   "app": {
     "background": {
       "scripts": ["test.js"]
     }
   }
 }