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
#include "shell/browser/api/electron_api_ontents.h"
#include <memory>
#include <set>
#include <string>
#include <utility>
#include <vector>
#include "base/message_loop/message_loop_current.h"
#include "base/no_destructor.h"
#include "base/optional.h"
#include "base/strings/utf_string_conversions.h"
#include "base/threading/thread_restrictions.h"
#include "base/threading/thread_task_runner_handle.h"
#include "base/values.h"
#include "chrome/browser/browser_process.h"
#include "chrome/browser/ssl/security_state_tab_helper.h"
#include "content/browser/frame_host/frame_tree_node.h"             
#include "content/browser/frame_host/render_frame_host_manager.h"   
#include "content/browser/renderer_host/render_widget_host_impl.h"  
#include "content/browser/renderer_host/render_widget_host_view_base.h"  
#include "content/common/widget_messages.h"
#include "content/public/browser/child_process_security_policy.h"
#include "content/public/browser/download_request_utils.h"