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

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

 {
   "name": "Platform App Test: local iframes are allowed",
   "version": "1",
   "manifest_version": 2,
   "permissions": [
-    "experimental",
     "app.window"
   ],
   "app": {
     "background": {
       "scripts": ["test.js"]
     }
   }
 }