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
bool proceed,
                                    bool* proceed_to_fire_unload) {
  if (type_ == BROWSER_WINDOW || type_ == OFF_SCREEN)
    *proceed_to_fire_unload = proceed;
  else
    *proceed_to_fire_unload = true;
}
void WebContents::MoveContents(content::WebContents* source,
                               const gfx::Rect& pos) {
  Emit("move", pos);
}
void WebContents::CloseContents(content::WebContents* source) {
  Emit("close");
  if ((type_ == BROWSER_WINDOW || type_ == OFF_SCREEN) && owner_window())
    owner_window()->CloseContents(source);
}
void WebContents::ActivateContents(content::WebContents* source) {
  Emit("activate");
}
void WebContents::UpdateTargetURL(content::WebContents* source,
                                  const GURL& url) {
  Emit("update-target-url", url);
}
bool WebContents::IsPopupOrPanel(const content::WebContents* source) const {