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
const content::WebContents* web_contents) {
  auto context = web_contents->GetBrowserContext();
  auto site_instance = web_contents->GetSiteInstance();
  if (!context || !site_instance)
    return nullptr;
  auto storage_partition =
      content::BrowserContext::GetStoragePartition(context, site_instance);
  if (!storage_partition)
    return nullptr;
  return storage_partition->GetServiceWorkerContext();
}
void OnCapturePageDone(base::Callback<void(const gfx::Image&)> callback,
                       const SkBitmap& bitmap,
                       content::ReadbackResponse response) {
  callback.Run(gfx::Image::CreateFrom1xBitmap(bitmap));
}
}  
WebContents::WebContents(v8::Isolate* isolate,
                         content::WebContents* web_contents)
    : content::WebContentsObserver(web_contents),
      embedder_(nullptr),
      type_(REMOTE),
      request_id_(0),
      background_throttling_(true) {
  web_contents->SetUserAgentOverride(GetBrowserContext()->GetUserAgent());
  Init(isolate);
  AttachAsUserData(web_contents);
}
WebContents::WebContents(v8::Isolate* isolate,