448638: HtmlUtility.cleanupCss throws
java.util.ConcurrentModificationException

Change-Id: I4623957fa711a20bd2d913f43eaf5da6f77c6a52
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=448638
Reviewed-on: https://git.eclipse.org/r/35474
Tested-by: Hudson CI
Reviewed-by: Christoph Schwank <christoph.schwank@bsiag.com>
(cherry picked from commit 95658f77b3bd6647958024194830ba45fbac520f)
Reviewed-on: https://git.eclipse.org/r/35567