Decouple ScriptPromise creation from ScriptPromiseResolver.

It is error-prone that ScriptPromiseResolver has ScriptPromise and
users must explicitly call detachPromise() to detach it.
Now ScriptPromiseResolver does NOT have ScriptPromise from users perspective.

Also replace deprecated |fulfill| with |resolve|.

BUG=304154

Review URL: https://codereview.chromium.org/26004002

git-svn-id: svn://svn.chromium.org/blink/trunk@159698 bbb929c8-8fbe-4397-9dbb-9b2b20218538