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
.SetMethod("detachFromOuterFrame", &Ws::DetachFromOuterFrame)
      .SetMethod("isOffscreen", &Ws::IsOffScreen)
#if BUILDFLAG(ENABLE_OSR)
      .SetMethod("startPainting", &Ws::StartPainting)
      .SetMethod("stopPainting", &Ws::StopPainting)
      .SetMethod("isPainting", &Ws::IsPainting)
      .SetMethod("_setFrameRate", &Ws::SetFrameRate)
      .SetMethod("_getFrameRate", &Ws::GetFrameRate)
      .SetProperty("frameRate", &Ws::GetFrameRate,
                   &Ws::SetFrameRate)
#endif
      .SetMethod("invalidate", &Ws::Invalidate)
      .SetMethod("_setZoomLevel", &Ws::SetZoomLevel)
      .SetMethod("_getZoomLevel", &Ws::GetZoomLevel)
      .SetProperty("zoomLevel", &Ws::GetZoomLevel,
                   &Ws::SetZoomLevel)
      .SetMethod("_setZoomFactor", &Ws::SetZoomFactor)
      .SetMethod("_getZoomFactor", &Ws::GetZoomFactor)
      .SetProperty("zoomFactor", &Ws::GetZoomFactor,
                   &Ws::SetZoomFactor)
      .SetMethod("getType", &Ws::GetType)
      .SetMethod("_getPreloadPaths", &Ws::GetPreloadPaths)
      .SetMethod("getWebPreferences", &Ws::GetWebPreferences)
      .SetMethod("getLastWebPreferences", &Ws::GetLastWebPreferences)
      .SetMethod("getOwnerBrowserWindow", &Ws::GetOwnerBrowserWindow)
      .SetMethod("inspectServiceWorker", &Ws::InspectServiceWorker)
      .SetMethod("inspectSharedWorker", &Ws::InspectSharedWorker)
      .SetMethod("inspectSharedWorkerById",
                 &Ws::InspectSharedWorkerById)
      .SetMethod("getAllSharedWorkers", &Ws::GetAllSharedWorkers)