Fix the issue File Download Crash in Debug mode

Crosswalk hits DCHECK_NE(content::DownloadItem::kInvalidId, id) failed, since
chromium upstream improved the code, we needed to override the virtual function
GetNextId().
Please see: https://codereview.chromium.org/15584002
This CL also fixed FileDownload error in Linux trybots.

https://github.com/crosswalk-project/crosswalk/issues/888
Merge pull request #896 from yinli/FileDownload

Fix the issue File Download Crash in Debug mode