Check for warnings when loading extensions in browser tests.

chrome/test/data/extensions/api_test/stubs/manifest.json

 {
   "name": "Content Script Extension API Stubs Test",
   "version": "1.0",
-  "background_page": "background.html",
+  "manifest_version": 2,
+  "background": {
+    "scripts": ["background.js"]
+  },
   "content_scripts": [
     {
       "matches": ["http://*/*"],
       "js":["content_script.js"]
     }
   ]
 }