Adding content_security_policy to sample extensions when relevant.

chrome/common/extensions/docs/examples/api/bookmarks/basic/manifest.json

 {
   "name": "My Bookmarks",
   "version": "1.0",
   "description": "A browser action with a popup dump of all bookmarks, including search, add, edit and delete.",
   "permissions": [
     "bookmarks", "tabs"
   ],
   "browser_action": {
       "default_title": "My Bookmarks.",
       "default_icon": "icon.png",
       "popup": "popup.html"
-  }
-}
+  },
+  "content_security_policy": "default-src 'self'"
+}