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_output_device.cc
#include "atom/browser/osr/osr_output_device.h"
#include "third_party/skia/include/core/SkDevice.h"
#include "ui/gfx/skia_util.h"
namespace atom {
OffScreenOutputDevice::OffScreenOutputDevice(bool transparent,
                                             const OnPaintCallback& callback)
  : transparent_(transparent),
    callback_(callback),
    active_(false) {
  DCHECK(!callback_.is_null());
}
OffScreenOutputDevice::~OffScreenOutputDevice() {