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.cc
std::unique_ptr<content::RenderWidgetHostViewFrameSubscriber> subscriber) {
  delegated_frame_host_->BeginFrameSubscription(std::move(subscriber));
}
void OffScreenRenderWidgetHostView::EndFrameSubscription() {
  delegated_frame_host_->EndFrameSubscription();
}
bool OffScreenRenderWidgetHostView::HasAcceleratedSurface(const gfx::Size &) {
  return false;
}
void OffScreenRenderWidgetHostView::GetScreenInfo(
    blink::WebScreenInfo* results) {
  results->rect = gfx::Rect(size_);
  results->availableRect = gfx::Rect(size_);
  results->depth = 24;
  results->depthPerComponent = 8;
  results->deviceScaleFactor = scale_factor_;
  results->orientationAngle = 0;
  results->orientationType = blink::WebScreenOrientationLandscapePrimary;
}
bool OffScreenRenderWidgetHostView::GetScreenColorProfile(
    blink::WebVector<char>*) {
  return false;
}
gfx::Rect OffScreenRenderWidgetHostView::GetBoundsInRootWindow() {
  return gfx::Rect(size_);
}
void OffScreenRenderWidgetHostView::LockCompositingSurface() {
}
void OffScreenRenderWidgetHostView::UnlockCompositingSurface() {