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
Enable Aura build on all platforms except Android

Enable Aura build on all platforms except Android.
Merge pull request #897 from pozdnyakov/Enable_Aura_except_Android

Enable Aura build on all platforms except Android
[Extensions] Test extensions with nested names

Add a test for nested extension names. This models our use case for
Tizen-Extensions-Crosswalk, that have the "tizen" extension as well as
many "tizen.*" subextensions.

The second test is disabled, but will serve to check whether we load a
"leaf" extension on demand.
Merge pull request #890 from cmarcelo/nested-test

[Extensions] Test extensions with nested names
Merge pull request #896 from yinli/FileDownload

Fix the issue File Download Crash in Debug mode
Bump version to 2.31.18.0
[Tizen] Screen rotation

Rotate crosswalk windows when the rotation of device is changed.
This is not the W3C screen orientation API implementation. But the
API implementation will be based on this change to offer lock/unlock
functionalities.