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)
yup just like I mentioned above,   at least Athena, Content Shell and Chromecast use the "TestScreen" class not exactly for testing but production instead. So I think we are fine on using it as long they are using it as well. I've added a comment now explaining that though.