Add ASSERT() to avoid accidental leaking ServiceWorkerContainer to cross origin context.

BUG=522791

Review URL: https://codereview.chromium.org/1305903007

git-svn-id: svn://svn.chromium.org/blink/trunk@201889 bbb929c8-8fbe-4397-9dbb-9b2b20218538
NavigatorServiceWorker* NavigatorServiceWorker::toNavigatorServiceWorker(Navigator& navigator)
{
    return static_cast<NavigatorServiceWorker*>(HeapSupplement<Navigator>::from(navigator, supplementName()));
}

NavigatorServiceWorker* NavigatorServiceWorker::toNavigatorServiceWorker(Navigator& navigator)
{
    return static_cast<NavigatorServiceWorker*>(HeapSupplement<Navigator>::from(navigator, supplementName()));
}
