Mark perf/append-text-nodes-without-renderers.html as slow.

TBR=dpranke@chromium.org

Review URL: https://codereview.chromium.org/26846005

git-svn-id: svn://svn.chromium.org/blink/trunk@159380 bbb929c8-8fbe-4397-9dbb-9b2b20218538