Revoke object URL to avoid ImagePlayer's leak. Fix #706.

From the [Sample Code](https://developer.mozilla.org/en-US/docs/Using_files_from_web_applications#Example.3A_Using_object_URLs_to_display_images) for MDN, we could release object url right after image loaded to avoid leak.
`p.then(..)` does always return a resolved promise, but we return p here and the p.then's result would not affact that of p.