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)
noupe. At least not using the way that ApplyWindowDefaultParams is implemented right now, with create_params.bounds being initialized with a default value of 840, 600. Even if I force the screen to be initialized like FULLSCREEN_FOR_LAUNCH, create_params.bounds will contain the default value which is not really fullscreen (the way we want).

That said, we probably could improve a bit how the things work here and try to make FULLSCREEN_FOR_LAUNCH work in fact but I rather wait until the final solution in upstream gets sorted out (i.e. to move ui::DisplayConfigurator out from ChromeOS to a common place).