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 "atom/browser/api/atom_api_web_contents.h"
#include <set>
#include <string>
#include "atom/browser/api/atom_api_debugger.h"
#include "atom/browser/api/atom_api_session.h"
#include "atom/browser/api/atom_api_window.h"
#include "atom/browser/atom_browser_client.h"
#include "atom/browser/atom_browser_context.h"
#include "atom/browser/atom_browser_main_parts.h"
#include "atom/browser/atom_security_state_model_client.h"
#include "atom/browser/lib/bluetooth_chooser.h"
#include "atom/browser/native_window.h"
#include "atom/browser/net/atom_network_delegate.h"
#include "atom/browser/osr/osr_output_device.h"
#include "atom/browser/osr/osr_web_contents_view.h"
#include "atom/browser/osr/osr_render_widget_host_view.h"
#include "atom/browser/ui/drag_util.h"
#include "atom/browser/web_contents_permission_helper.h"