Implement ScriptPromise::then

This is basically an attempt to revive once abandoned patch:
https://codereview.chromium.org/23254004

BUG=337098
TEST=fast/js/Promise-native-then.html

Review URL: https://codereview.chromium.org/145543002

git-svn-id: svn://svn.chromium.org/blink/trunk@165730 bbb929c8-8fbe-4397-9dbb-9b2b20218538