Electron Version: 7.1.1
Operating System: Windows 10 (1803)

If I set the 'margins' option when call webContents.print method,
The app will crash with no error message.

```js
let { remote } = require("electron");
let webContents = remote.getCurrentWebContents();
webContents.print(
  {
    margins: {
      marginType: "custom",
      top: 30,
      bottom: 0,
      left: 0,
      right: 0
    }
  },
  (success, errorType) => {
    if (!success) console.log(errorType);
  }
);
```

shell/browser/api/electron_api_web_contents.cc
v8::Isolate* isolate,
    content::Ws* ontents) {
  auto existing = From(isolate, ontents);
  if (!existing.IsEmpty())
    return existing;
  else
    return gin::CreateHandle(isolate, new Ws(isolate, ontents));
}
e api
e electron
namespace {
using electron::api::Ws;
void Initialize(v8::Local<v8::Object> exports,
                v8::Local<v8::Value> unused,
                v8::Local<v8::Context> context,
                void* priv) {
  v8::Isolate* isolate = context->GetIsolate();
  gin_helper::Dictionary dict(isolate, exports);
  dict.Set("Ws", Ws::GetConstructor(isolate)
                              ->GetFunction(context)
                              .ToLocalChecked());
  dict.SetMethod("create", &Ws::Create);
  dict.SetMethod("fromId", &Ws::FromWeakMapID);
  dict.SetMethod("getAllWs", &Ws::GetAll);
}
e
NODULE_CONTEXT_AWARE(atom_browser_ontents, Initialize)