Mark fast/dom/element-attribute-js-null.html as Leak.

BUG=403987
TBR=kouhei@chromium.org

Review URL: https://codereview.chromium.org/472193002

git-svn-id: svn://svn.chromium.org/blink/trunk@180308 bbb929c8-8fbe-4397-9dbb-9b2b20218538