Remove ScopedPersistent::get usage in ScriptPromiseResolver(Test)?.

It's going to be removed soon.

BUG=236290

Review URL: https://chromiumcodereview.appspot.com/19365003

git-svn-id: svn://svn.chromium.org/blink/trunk@154406 bbb929c8-8fbe-4397-9dbb-9b2b20218538