generalize CallbacksPromiseAdapter

This generalizes a class that used to be ServiceWorker-specific, that adapts WebCallbacks objects into script-friendly objects, for use in ScriptPromiseResolver and friends.

BUG=

Review URL: https://codereview.chromium.org/26901006

git-svn-id: svn://svn.chromium.org/blink/trunk@159742 bbb929c8-8fbe-4397-9dbb-9b2b20218538