Add support for CallbackPromiseAdapter<void, T> matching WebCallbacks<void, T>.

This makes it possible to eliminate the various UndefinedValue classes that are sprinkled throughout the Service Worker code.

Review URL: https://codereview.chromium.org/468913007

git-svn-id: svn://svn.chromium.org/blink/trunk@180703 bbb929c8-8fbe-4397-9dbb-9b2b20218538