Implement Promise.cast

Implement Promise.cast static method, it is newly introduced by the
latest Promise draft[1].

[1]: https://github.com/domenic/promises-unwrapping

BUG=295420

Review URL: https://chromiumcodereview.appspot.com/24506002

git-svn-id: svn://svn.chromium.org/blink/trunk@158365 bbb929c8-8fbe-4397-9dbb-9b2b20218538