Remove flaky timeout expectation for compositing/culling/clear-fixed-iframe.html

It consistently passed for a while now

BUG=245203
TBR=noel@chromium.org

Review URL: https://codereview.chromium.org/19595002

git-svn-id: svn://svn.chromium.org/blink/trunk@154415 bbb929c8-8fbe-4397-9dbb-9b2b20218538