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::AtomBrowserClient::SuppressRendererProcessRestartForOnce();
  web_contents()->GetController().GoForward();
}
void WebContents::GoToOffset(int offset) {
  atom::AtomBrowserClient::SuppressRendererProcessRestartForOnce();
  web_contents()->GetController().GoToOffset(offset);
}
bool WebContents::IsCrashed() const {
  return web_contents()->IsCrashed();
}
void WebContents::SetUserAgent(const std::string& user_agent,
                               mate::Arguments* args) {
  web_contents()->SetUserAgentOverride(user_agent);
}
std::string WebContents::GetUserAgent() {
  return web_contents()->GetUserAgentOverride();
}
void WebContents::InsertCSS(const std::string& css) {
  web_contents()->InsertCSS(css);
}
bool WebContents::SavePage(const base::FilePath& full_file_path,
                           const content::SavePageType& save_type,
                           const SavePageHandler::SavePageCallback& callback) {
  auto handler = new SavePageHandler(web_contents(), callback);
  return handler->Handle(full_file_path, save_type);
}
void WebContents::OpenDevTools(mate::Arguments* args) {