Adding content_security_policy to sample extensions when relevant.

chrome/common/extensions/docs/examples/api/pageAction/pageaction_by_content/manifest.json

 {
   "name" : "Page action by content",
   "version" : "1.0",
   "description" : "Shows a page action for HTML pages containing the word 'sandwich'",
   "background_page" : "background.html",
   "page_action" :
   {
     "default_icon" : "sandwich-19.png",
     "default_title" : "There's a 'sandwich' in this page!"
   },
   "content_scripts" : [
     {
       "matches" : [
         "http://*/*",
         "https://*/*"
       ],
       "js" : ["contentscript.js"],
       "run_at" : "document_idle",
       "all_frames" : false
     }
   ],
   "icons" : {
     "48" : "sandwich-48.png",
     "128" : "sandwich-128.png"
-  }
+  },
+  "content_security_policy": "default-src 'self'"
 }