Use v8::Promise::Then in ScriptPromise::then.

ScriptPromise::then should use v8::Promise::Then in order to handle
multiply-wrapped Promises correctly.

BUG=371288

Review URL: https://codereview.chromium.org/316453002

git-svn-id: svn://svn.chromium.org/blink/trunk@176194 bbb929c8-8fbe-4397-9dbb-9b2b20218538