updated test expectation: editing/spelling/spellcheck-editable-on-focus.html flaky

BUG=278318
TBR=jianli@chromium.org

Review URL: https://codereview.chromium.org/22880037

git-svn-id: svn://svn.chromium.org/blink/trunk@156638 bbb929c8-8fbe-4397-9dbb-9b2b20218538