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
v8::Local<v8::Value> val,
                     content::SavePageType* out) {
    std::string save_type;
    if (!ConvertFromV8(isolate, val, &save_type))
      return false;
    save_type = base::ToLowerASCII(save_type);
    if (save_type == "htmlonly") {
      *out = content::SAVE_PAGE_TYPE_AS_ONLY_HTML;
    } else if (save_type == "htmlcomplete") {
      *out = content::SAVE_PAGE_TYPE_AS_COMPLETE_HTML;
    } else if (save_type == "mhtml") {
      *out = content::SAVE_PAGE_TYPE_AS_MHTML;
    } else {
      return false;
    }
    return true;
  }
};
template <>
struct Converter<electron::api::Ws::Type> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,