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/osr/osr_render_widget_host_view.h
#include <windows.h>
#endif
#include "atom/browser/native_window.h"
#include "atom/browser/osr/osr_output_device.h"
#include "base/process/kill.h"
#include "base/threading/thread.h"
#include "base/time/time.h"
#include "cc/scheduler/begin_frame_source.h"
#include "cc/output/compositor_frame.h"
#include "content/browser/renderer_host/delegated_frame_host.h"
#include "content/browser/renderer_host/render_widget_host_impl.h"
#include "content/browser/renderer_host/render_widget_host_view_base.h"
#include "content/browser/renderer_host/resize_lock.h"
#include "ui/compositor/compositor.h"
#include "ui/compositor/layer_delegate.h"
#include "ui/compositor/layer_owner.h"
#include "ui/base/ime/text_input_client.h"
#include "ui/gfx/geometry/point.h"
#include "third_party/WebKit/public/platform/WebVector.h"
#if defined(OS_WIN)
#include "ui/gfx/win/window_impl.h"
#endif
#if defined(OS_MACOSX)
#include "content/browser/renderer_host/browser_compositor_view_mac.h"
#include "ui/accelerated_widget_mac/accelerated_widget_mac.h"
#endif
#if defined(OS_MACOSX)
#ifdef __OBJC__
@class CALayer;
@class NSWindow;