chrome/browser/resources/feedback/js/take_screenshot.js

// Copyright 2013 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.

/**
 * Function to take the screenshot of the current screen.
 * @param {function(HTMLCanvasElement)} callback Callback for returning the
 *                                      canvas with the screenshot on it.
 */
function takeScreenshot(callback) {
  var screenshotStream = null;
  var video = document.createElement('video');

  video.addEventListener('canplay', function(e) {
    if (screenshotStream) {
      var canvas = document.createElement('canvas');
      canvas.setAttribute('width', video.videoWidth);
      canvas.setAttribute('height', video.videoHeight);
      canvas.getContext('2d').drawImage(
          video, 0, 0, video.videoWidth, video.videoHeight);

      video.pause();
      video.src = '';

      screenshotStream.stop();
      screenshotStream = null;

      callback(canvas);
    }
  }, false);

  navigator.webkitGetUserMedia(
    {
      video: {
        mandatory: {
          chromeMediaSource: 'screen',
          maxWidth: 4096,
          maxHeight: 2560
        }
      }
    },
    function(stream) {
      if (stream) {
        screenshotStream = stream;
        video.src = window.URL.createObjectURL(screenshotStream);
        video.play();
      }
    },
    function(err) {
      console.error('takeScreenshot failed: ' +
          err.name + '; ' + err.message + '; ' + err.constraintName);
    }
  );
}