Updating attributeChangedCallback support to include new value.

This follows the spec at https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/custom/index.html#dfn-attribute-changed-callback

Also ensuring that remove passes null as the new value, rather than undefined.

BUG=
R=jmesserly@google.com

Review URL: https://codereview.appspot.com/14154044