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 snip was copied from content/shell/browser/shell_platform_data_aura.cc. I think we are fine on using it as long content_shell is using as well... but yeah, definitely a bad name for a class.