WebIDL operator functions should not have a prototype property

The functions used for DOM methods (like Node.prototype.appendChild)
should not have a "prototype" property.

This makes us match Firefox, IE and Safari.

BUG=272440

Review URL: https://chromiumcodereview.appspot.com/23531007

git-svn-id: svn://svn.chromium.org/blink/trunk@156996 bbb929c8-8fbe-4397-9dbb-9b2b20218538