Make test more robust as to when layout happens.

This fix is necessary so a Chrome change I'd like to land doesn't break the
test.

BUG=393829
NOTRY=true

Review URL: https://codereview.chromium.org/477293003

git-svn-id: svn://svn.chromium.org/blink/trunk@180660 bbb929c8-8fbe-4397-9dbb-9b2b20218538