[Mac] Don't use Aura for Mac

Aura is not ready for Mac currently. It has not been enabled for Mac in upstream.
Thus, it fails even during 'gclient sync' due to some duplicate keys in resource files.

More details:

https://crosswalk-project.org/jira/browse/XWALK-85
Merge pull request #1018 from poussa/mac-gclient-sync

[Mac] Don't use Aura for Mac