Fix Issue 301 from https://code.google.com/p/gwt-platform/

Fix issue 301 (PresenterWidget is always centered with RevealRootPopupContentEvent) from https://code.google.com/p/gwt-platform/issues/detail?id=301 by merging old patch from https://codereview.appspot.com/5988064/ into a pull request.
Additionally the duplicate JavaDoc comment from RevealRootPopupContentEvent was merged into one comment.