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
content::WebContents::CreateParams params(session->browser_context());
    web_contents = content::WebContents::Create(params);
  }
  Observe(web_contents);
  InitWithWebContents(web_contents, session->browser_context());
  managed_web_contents()->GetView()->SetDelegate(this);
  
  new WebContentsPreferences(web_contents, options);
  
  WebContentsPermissionHelper::CreateForWebContents(web_contents);
  
  AtomSecurityStateModelClient::CreateForWebContents(web_contents);
  web_contents->SetUserAgentOverride(GetBrowserContext()->GetUserAgent());
  if (IsGuest()) {
    guest_delegate_->Initialize(this);
    NativeWindow* owner_window = nullptr;
    if (options.Get("embedder", &embedder_) && embedder_) {
      
      auto relay =
          NativeWindowRelay::FromWebContents(embedder_->web_contents());
      if (relay)
        owner_window = relay->window.get();
    }
    if (owner_window)
      SetOwnerWindow(owner_window);
  }
  Init(isolate);
  AttachAsUserData(web_contents);
}
WebContents::~WebContents() {