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
if (managed_ontents())
    managed_ontents()->SetDevToolsWs(devtools->ontents());
}
v8::Local<v8::Value> Ws::GetNativeView() const {
  gfx::NativeView ptr = ontents()->GetNativeView();
  auto buffer = node::Buffer::Copy(isolate(), reinterpret_cast<char*>(&ptr),
                                   sizeof(gfx::NativeView));
  if (buffer.IsEmpty())
    return v8::Null(isolate());
  else
    return buffer.ToLocalChecked();
}
v8::Local<v8::Value> Ws::DevToolsWs(v8::Isolate* isolate) {
  if (devtools_ontents_.IsEmpty())
    return v8::Null(isolate);
  else
    return v8::Local<v8::Value>::New(isolate, devtools_ontents_);
}
v8::Local<v8::Value> Ws::Debugger(v8::Isolate* isolate) {