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.h
}
namespace mate {
class Arguments;
class Dictionary;
}
namespace atom {
struct SetSizeParams;
class AtomBrowserContext;
class WebViewGuestDelegate;
namespace api {
class WebContents : public mate::TrackableObject<WebContents>,
                    public CommonWebContentsDelegate,
                    public content::WebContentsObserver {
 public:
  enum Type {
    BACKGROUND_PAGE,  
    BROWSER_WINDOW,  
    REMOTE,  
    WEB_VIEW,  
    OFF_SCREEN,  
  };
  
  using PrintToPDFCallback =
      base::Callback<void(v8::Local<v8::Value>, v8::Local<v8::Value>)>;