Add compositing/rtl/rtl-iframe-fixed-overflow.html to existing bug

TBR=wangxianzhu@chromium.org
BUG=338794,192172

Review URL: https://codereview.chromium.org/346473003

git-svn-id: svn://svn.chromium.org/blink/trunk@176703 bbb929c8-8fbe-4397-9dbb-9b2b20218538