Adding content_security_policy to sample extensions when relevant.

chrome/common/extensions/docs/examples/howto/contentscript_xhr/manifest.json

 {
   "name": "Content Script Cross-Domain XMLHttpRequest Example",
   "version": "2.0.0",
   "description": "Demonstrates making cross domain requests from a content script by putting Twitter trends on Google News.",
   "permissions": [
     "https://api.twitter.com/*"
   ],
   "icons": {
     "48" : "sample-48.png",
     "128" : "sample-128.png"
   },
   "content_scripts": [
     {
       "matches": ["http://news.google.com/*"],
       "js" : ["contentscript.js"]
     }
-  ]
+  ],
+  "content_security_policy": "default-src 'self' https://api.twitter.com http://news.google.com"
 }