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
if (type_ == REMOTE)
    return;
  std::string state;
  if (type_ == WEB_VIEW || !owner_window()) {
    state = "detach";
  } else if (args && args->Length() == 1) {
    bool detach = false;
    mate::Dictionary options;
    if (args->GetNext(&options)) {
      options.Get("mode", &state);
      options.Get("detach", &detach);
      if (state.empty() && detach)
        state = "detach";
    }
  }
  managed_web_contents()->SetDockState(state);
  managed_web_contents()->ShowDevTools();
}
void WebContents::CloseDevTools() {
  if (type_ == REMOTE)
    return;
  managed_web_contents()->CloseDevTools();
}
bool WebContents::IsDevToolsOpened() {
  if (type_ == REMOTE)
    return false;
  return managed_web_contents()->IsDevToolsViewShowing();
}
bool WebContents::IsDevToolsFocused() {