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
atom::api::WebContents::Type* out) {
    using Type = atom::api::WebContents::Type;
    std::string type;
    if (!ConvertFromV8(isolate, val, &type))
      return false;
    if (type == "webview") {
      *out = Type::WEB_VIEW;
    } else if (type == "backgroundPage") {
      *out = Type::BACKGROUND_PAGE;
    } else if (type == "offscreen") {
      *out = Type::OFF_SCREEN;
    } else {
      return false;
    }
    return true;
  }
};
}  
namespace atom {
namespace api {
namespace {
content::ServiceWorkerContext* GetServiceWorkerContext(