Adding content_security_policy to sample extensions when relevant.

chrome/common/extensions/docs/examples/api/messaging/timer/manifest.json

 {
   "name": "Message Timer",
   "version": "1.0",
   "description": "Times how long it takes to send a message to a content script and back.",
   "permissions": ["tabs"],
   "content_scripts": [
     {
       "matches": ["http://*/*"],
       "js": ["page.js"]
     }
   ],
   "browser_action": {
     "default_title": "Time to current page",
     "default_icon": "clock.png",
     "popup": "popup.html"
-  }
+  },
+  "content_security_policy": "default-src 'self'"
 }