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
[Runtime] Add Manifest keys handler mechanism

The key handler can parse specific member in manifest.json for more
information, and save these information into the Application object, by
this, it will be easier to use without parsing these information every
time. The mechanism also can treat dependence relationship between
Manifest keys, it make sure that a high priority key will be always
parsed before a low one, means that when parsing the low priority key,
all required information has been prepared yet.

The key handler also can validate if the key value is allowed for this
application.

All keys handlers should be registered in
RegisterManifestHandlers().