Remove the Promises old implementation.

As V8 Promises are enabled, the old Promises implementation can be deleted.
This CL also adds some unittests for ScriptPromise and ScriptPromiseResolver.

BUG=352552, 352597

Review URL: https://codereview.chromium.org/293933002

git-svn-id: svn://svn.chromium.org/blink/trunk@174632 bbb929c8-8fbe-4397-9dbb-9b2b20218538