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 web_contents()->GetRenderProcessHost()->GetID();
}
WebContents::Type WebContents::GetType() const {
  return type_;
}
bool WebContents::Equal(const WebContents* web_contents) const {
  return GetID() == web_contents->GetID();
}
void WebContents::LoadURL(const GURL& url, const mate::Dictionary& options) {
  if (!url.is_valid()) {
    Emit("did-fail-load",
         static_cast<int>(net::ERR_INVALID_URL),
         net::ErrorToShortString(net::ERR_INVALID_URL),
         url.possibly_invalid_spec(),
         true);
    return;
  }
  content::NavigationController::LoadURLParams params(url);
  GURL http_referrer;
  if (options.Get("httpReferrer", &http_referrer))
    params.referrer = content::Referrer(http_referrer.GetAsReferrer(),
                                        blink::WebReferrerPolicyDefault);
  std::string user_agent;
  if (options.Get("userAgent", &user_agent))
    web_contents()->SetUserAgentOverride(user_agent);
  std::string extra_headers;
  if (options.Get("extraHeaders", &extra_headers))
    params.extra_headers = extra_headers;