Fix empty-textarea-toggle-disabled.html flaky timeouts.

It doesn't look like the test needs to be async.

BUG=252573

Review URL: https://codereview.chromium.org/50643002

git-svn-id: svn://svn.chromium.org/blink/trunk@160857 bbb929c8-8fbe-4397-9dbb-9b2b20218538