Crosswalk embedded support

per request of @ilkkasyrjala I'm pushing the following changes for review, which gives embedded support for Crosswalk on Linux. PTAL @ds-hwang @pozdnyakov. (CC-ing @baleboy @rakuco @poussa @tmpsantos)
Question: You're using API of aura::test, I'm afraid they are only for testing. Please correct me if I'm wrong.