Widen expectations for editing/spelling/spellcheck-editable-on-focus.html

Now also timing out on Mac/Debug, and it's consistently timing out,
not flakily.

BUG=356957
TBR=mkwst@chromium.org

Review URL: https://codereview.chromium.org/476843004

git-svn-id: svn://svn.chromium.org/blink/trunk@180301 bbb929c8-8fbe-4397-9dbb-9b2b20218538