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)
this is not overriden virtual, two questions
1) it's body is empty, do we need it? (how it should be used?)
2) who is gonna override it? (why is it virtual?)