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
&Ws::SetBackgroundThrottling)
      .SetMethod("getProcessId", &Ws::GetProcessID)
      .SetMethod("getOSProcessId", &Ws::GetOSProcessID)
      .SetMethod("_getOSProcessIdForFrame",
                 &Ws::GetOSProcessIdForFrame)
      .SetMethod("equal", &Ws::Equal)
      .SetMethod("_loadURL", &Ws::LoadURL)
      .SetMethod("downloadURL", &Ws::DownloadURL)
      .SetMethod("_getURL", &Ws::GetURL)
      .SetMethod("getTitle", &Ws::GetTitle)
      .SetMethod("isLoading", &Ws::IsLoading)
      .SetMethod("isLoadingMainFrame", &Ws::IsLoadingMainFrame)
      .SetMethod("isWaitingForResponse", &Ws::IsWaitingForResponse)
      .SetMethod("_stop", &Ws::Stop)
      .SetMethod("_goBack", &Ws::GoBack)
      .SetMethod("_goForward", &Ws::GoForward)
      .SetMethod("_goToOffset", &Ws::GoToOffset)
      .SetMethod("isCrashed", &Ws::IsCrashed)
      .SetMethod("_setUserAgent", &Ws::SetUserAgent)
      .SetMethod("_getUserAgent", &Ws::GetUserAgent)
      .SetProperty("userAgent", &Ws::GetUserAgent,
                   &Ws::SetUserAgent)
      .SetMethod("savePage", &Ws::SavePage)
      .SetMethod("openDevTools", &Ws::OpenDevTools)
      .SetMethod("closeDevTools", &Ws::CloseDevTools)
      .SetMethod("isDevToolsOpened", &Ws::IsDevToolsOpened)
      .SetMethod("isDevToolsFocused", &Ws::IsDevToolsFocused)
      .SetMethod("enableDeviceEmulation", &Ws::EnableDeviceEmulation)
      .SetMethod("disableDeviceEmulation", &Ws::DisableDeviceEmulation)
      .SetMethod("toggleDevTools", &Ws::ToggleDevTools)