Fixes a memory leak in ScrollImplTrident by removing references from elements to GWT code.

http://gwt-code-reviews.appspot.com/1601803/

Issue: 7015
Author: stephen.haberman, dirk.scheffler
Review: jlabanca

Review by: rdayal@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10823 8db76d5a-ed1c-0410-87a9-c151d255dfc7