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
bitmap_size = gfx::ScaleToCeiledSize(view_size, scale);
  host->CopyFromBackingStore(gfx::Rect(rect.origin(), view_size),
                             bitmap_size,
                             base::Bind(&OnCapturePageDone, callback),
                             kBGRA_8888_SkColorType);
}
void WebContents::OnCursorChange(const content::WebCursor& cursor) {
  content::WebCursor::CursorInfo info;
  cursor.GetCursorInfo(&info);
  if (cursor.IsCustom()) {
    Emit("cursor-changed", CursorTypeToString(info),
      gfx::Image::CreateFrom1xBitmap(info.custom_image),
      info.image_scale_factor,
      gfx::Size(info.custom_image.width(), info.custom_image.height()),
      info.hotspot);
  } else {
    Emit("cursor-changed", CursorTypeToString(info));
  }
}
void WebContents::SetSize(const SetSizeParams& params) {
  if (guest_delegate_)
    guest_delegate_->SetSize(params);
}
bool WebContents::IsGuest() const {
  return type_ == WEB_VIEW;
}
bool WebContents::IsOffScreen() const {
  return type_ == OFF_SCREEN;
}
void WebContents::OnPaint(const gfx::Rect& dirty_rect,