Make ServiceWorkerContainer Oilpan-ready.

After r177463, ScriptPromiseProperty holders need to have a 'trace'
method. ScriptPromiseProperty was added to implement
navigator.serviceWorker.ready, so now ServiceWorkerContainer needs to
have a 'trace' method. It seems simpler to get ServiceWorkerContainer
ready for Oilpan than it is to anachronistically specialize
ScriptPromiseProperty::trace to handle non-traceable holders.

BUG=363967

Review URL: https://codereview.chromium.org/365063008

git-svn-id: svn://svn.chromium.org/blink/trunk@177520 bbb929c8-8fbe-4397-9dbb-9b2b20218538