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
#include "content/public/browser/plugin_service.h"
#include "content/public/browser/render_frame_host.h"
#include "content/public/browser/render_process_host.h"
#include "content/public/browser/render_view_host.h"
#include "content/public/browser/render_widget_host.h"
#include "content/public/browser/render_widget_host_view.h"
#include "content/public/browser/resource_request_details.h"
#include "content/public/browser/service_worker_context.h"
#include "content/public/browser/storage_partition.h"
#include "content/public/browser/site_instance.h"
#include "content/public/browser/web_contents.h"
#include "content/public/common/context_menu_params.h"
#include "native_mate/dictionary.h"
#include "native_mate/object_template_builder.h"
#include "net/http/http_response_headers.h"
#include "net/url_request/url_request_context.h"
#include "third_party/WebKit/public/web/WebInputEvent.h"
#include "third_party/WebKit/public/web/WebFindOptions.h"
#include "ui/display/screen.h"
#include "atom/common/node_includes.h"
namespace {
struct PrintSettings {
  bool silent;
  bool print_background;
};
}  
namespace mate {
template<>
struct Converter<atom::SetSizeParams> {
  static bool FromV8(v8::Isolate* isolate,