Lazy background page branding: "Transient Background Page". Rename

chrome/test/data/extensions/api_test/lazy_background_page/messaging/manifest.json

 {
   "name": "Lazy BG messaging test",
   "description": "Test that message passing starts the background page",
   "version": "1",
   "manifest_version": 2,
   "permissions": ["experimental"],
   "background": {
     "scripts": ["background.js"],
-    "persistent": false
+    "transient": true
   },
   "content_scripts": [
     {
       "matches": ["http://*/*"],
       "js": ["content.js"]
     }
   ]
 }