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
const std::string& frame_name,
                                 WindowOpenDisposition disposition) {
  if (type_ == BROWSER_WINDOW || type_ == OFF_SCREEN)
    Emit("-new-window", target_url, frame_name, disposition);
  else
    Emit("new-window", target_url, frame_name, disposition);
}
content::WebContents* WebContents::OpenURLFromTab(
    content::WebContents* source,
    const content::OpenURLParams& params) {
  if (params.disposition != CURRENT_TAB) {
    if (type_ == BROWSER_WINDOW || type_ == OFF_SCREEN)
      Emit("-new-window", params.url, "", params.disposition);
    else
      Emit("new-window", params.url, "", params.disposition);
    return nullptr;
  }
  
  if (Emit("will-navigate", params.url))
    return nullptr;
  return CommonWebContentsDelegate::OpenURLFromTab(source, params);
}
void WebContents::BeforeUnloadFired(content::WebContents* tab,