Fix issue #6724 - clearOpacity doesn't clear the "filter" property in IE8.

Review at: http://gwt-code-reviews.appspot.com/1615805
Patch by: stephenh

Review by: jlabanca@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10941 8db76d5a-ed1c-0410-87a9-c151d255dfc7