Adding `content_security_policy` to a few sample extensions

chrome/common/extensions/docs/examples/api/tabs/screenshot/background.js

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // To make sure we can uniquely identify each screenshot tab, add an id as a
 // query param to the url that displays the screenshot.
 var id = 100;
 
 function takeScreenshot() {
   chrome.tabs.captureVisibleTab(null, function(img) {
@@ skipping 36 matching lines @@
 }
 
 // Listen for a click on the camera icon.  On that click, take a screenshot.
 chrome.browserAction.onClicked.addListener(function(tab) {
   if (tab.url.match(/code.google.com/)) {
     takeScreenshot();
   } else {
     alert('This sample can only take screenshots of code.google.com pages');
   }
 });