Make w3cwrapper.js's done() match the semantics in testharness.js

This should fix any tests where done() can be called multiple times.

BUG=333880

Review URL: https://codereview.chromium.org/205893003

git-svn-id: svn://svn.chromium.org/blink/trunk@169720 bbb929c8-8fbe-4397-9dbb-9b2b20218538