Fix flaky textarea-autofocus-removal-while-focusing.html

The test became flaky because r165275 made autofocus handling asynchronous.

BUG=324707
TBR=keishi@chromium.org

Review URL: https://codereview.chromium.org/141433004

git-svn-id: svn://svn.chromium.org/blink/trunk@165431 bbb929c8-8fbe-4397-9dbb-9b2b20218538