When in the onclick
attribute you do not need to specify that it is Javascript.
<button type="button"
onclick="window.open('', '_self', ''); window.close();">Discard</button>
This should do it. In order to close it your page needs to be opened by the script, hence the window.open. Here is an article explaining this in detail:
If all else fails, you should also add a message asking the user to manually close the window, as there is no cross-browser solution for this, especially with older browsers such as IE 8.