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
const std::string& channel,
                          blink::CloneableMessage arguments) {
  TRACE_EVENT1("electron", "Ws::Message", "channel", channel);
  // ws.emit('-ipc-message', new Event(), internal, channel,
  // arguments);
  EmitWithSender("-ipc-message", bindings_.dispatch_context(), InvokeCallback(),
                 internal, channel, std::move(arguments));
}
void Ws::Invoke(bool internal,
                         const std::string& channel,
                         blink::CloneableMessage arguments,
                         InvokeCallback callback) {
  TRACE_EVENT1("electron", "Ws::Invoke", "channel", channel);
  // ws.emit('-ipc-invoke', new Event(), internal, channel, arguments);
  EmitWithSender("-ipc-invoke", bindings_.dispatch_context(),
                 std::move(callback), internal, channel, std::move(arguments));
}
void Ws::MessageSync(bool internal,
                              const std::string& channel,
                              blink::CloneableMessage arguments,
                              MessageSyncCallback callback) {
  TRACE_EVENT1("electron", "Ws::MessageSync", "channel", channel);
  // ws.emit('-ipc-message-sync', new Event(sender, message), internal,
  // channel, arguments);
  EmitWithSender("-ipc-message-sync", bindings_.dispatch_context(),
                 std::move(callback), internal, channel, std::move(arguments));
}
void Ws::MessageTo(bool internal,