Remove didDownloadData stub added for waiting on chromium changes

This CL should be landed after the chromium-side change[1] is landed.

This stub code is introduced to add a new argument to didDownloadData[2].
Since the chromium-side change is landed[1], the stub code should be removed.

[1]: https://chromiumcodereview.appspot.com/23696002/
[2]: https://chromiumcodereview.appspot.com/23632004/

BUG=269055

Review URL: https://chromiumcodereview.appspot.com/23567013

git-svn-id: svn://svn.chromium.org/blink/trunk@157190 bbb929c8-8fbe-4397-9dbb-9b2b20218538