I tired with the offscreen mode and here are the problems I found:
- [x] <del>The `data` of `paint` event is empty when loading simple local pages.</del>
- [x] The size of `data` is wrong, current implement uses the size of pointer (`sizeof(bitmap_pixels)`).
- [x] Crashes when opening devtools for offscreen window.
- [x] We should probably show a dummy view indicating it is offscreen instead of provide NULL.
- [x] The offscreen window is affecting the rendering of other windows. Try running `./script/test.py -g offscreen` and then refreshing the window.

/cc @gerhardberger @brenca

atom/browser/api/atom_api_web_contents.cc
return static_cast<AtomBrowserContext*>(web_contents()->GetBrowserContext());
}
void WebContents::OnRendererMessage(const base::string16& channel,
                                    const base::ListValue& args) {
  
  Emit(base::UTF16ToUTF8(channel), args);
}
void WebContents::OnRendererMessageSync(const base::string16& channel,
                                        const base::ListValue& args,
                                        IPC::Message* message) {
  
  EmitWithSender(base::UTF16ToUTF8(channel), web_contents(), message, args);
}
mate::Handle<WebContents> WebContents::CreateFrom(