Change ScriptPromise / ScriptPromiseResolver API.

In preparation for the V8 Promises adoption, change the ScriptPromise[Resolver] API.

BUG=352552
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/196383017

git-svn-id: svn://svn.chromium.org/blink/trunk@169243 bbb929c8-8fbe-4397-9dbb-9b2b20218538