Adding content_security_policy to sample extensions when relevant.

chrome/common/extensions/docs/examples/api/extension/isAllowedAccess/manifest.json

 {
   "name" : "`extension.isAllowed???Access` Example",
   "version" : "1.0.0",
   "description" : "Demonstrates the `extension.isAllowed???Access` APIs",
   "permissions" : [ "file://*" ],
   "browser_action" : {
     "default_popup": "popup.html",
     "default_icon" : "sample-19.png"
   },
   "icons" : {
     "16" : "sample-16.png",
     "48" : "sample-48.png",
     "128" : "sample-128.png"
-  }
+  },
+  "content_security_policy": "default-src 'self'"
 }