Key event listener produce different key codes in Chrome and Firefox

[Bjoern]

Steps to reproduce

1. Visit ​http://ckeditor.com/features in firefox and chrome
2. Enter the following into console:

   CKEDITOR.instances.ckdemo.on('key', function(event){
     console.log("key-event-data: ", event.data);
   });
   

3. Focus into CKEditor and press CTRL
4. Console output:
   in Firefox - keyCode: 17
   in Chrome - keyCode: 1114129

Expected result

In Firefox the keyCode result should be 1114129 (like CKEDITOR.CTRL)

Actual result

In Firefox the keyCode result is 17

Other details (browser, OS, CKEditor version, installed plugins)

CKEditor:

Actual demo version on homepage ​http://ckeditor.com/features (4.6.2)

Browser:

Firefox: 53.0 (64bit)
Chrome: 58.0.3029.96 (64-bit)

OS:

Ubuntu 16.10