Conversion of Promise is trivial

Promise values are converted using ScriptPromise::cast(), which is trivial
insofar as it can't cause an exception to be thrown.

BUG=413257

Review URL: https://codereview.chromium.org/606603002

git-svn-id: svn://svn.chromium.org/blink/trunk@182676 bbb929c8-8fbe-4397-9dbb-9b2b20218538