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)
okay I'll return NULL simply. One other idea is to rename color_chooser_aura.cc to color_chooser_views.cc and actually create a stubbed version of the real aura to be used here (that will have the same color_chooser_aura.cc name)