Move chrome.appWindow to chrome.app.window.

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

 {
   "name": "Platform App Test: various document, window, and API restrictions",
   "version": "1",
   "manifest_version": 2,
   "permissions": [
-    "experimental",
-    "appWindow"
+    "experimental"
   ],
   "app": {
     "background": {
       "scripts": ["test.js"]
     }
   },
   "sandbox": {
     "pages": ["sandboxed_iframe.html"]
   }
 }