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
#ifndef ATOM_BROWSER_API_ATOM_API_WEB_CONTENTS_H_
#define ATOM_BROWSER_API_ATOM_API_WEB_CONTENTS_H_
#include <string>
#include <vector>
#include "atom/browser/api/frame_subscriber.h"
#include "atom/browser/api/save_page_handler.h"
#include "atom/browser/api/trackable_object.h"
#include "atom/browser/common_web_contents_delegate.h"
#include "content/public/browser/web_contents_observer.h"
#include "content/public/common/favicon_url.h"
#include "content/common/cursors/webcursor.h"
#include "native_mate/handle.h"
#include "ui/gfx/image/image.h"
namespace blink {
struct WebDeviceEmulationParams;
}
namespace brightray {
class InspectableWebContents;