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::Isolate* isolate,
      const scoped_refptr<content::DevToolsAgentHost>& val) {
    gin_helper::Dictionary dict(isolate, v8::Object::New(isolate));
    dict.Set("id", val->GetId());
    dict.Set("url", val->GetURL().spec());
    return dict.GetHandle();
  }
};
}  
namespace electron {
namespace api {
namespace {
void OnCapturePageDone(gin_helper::Promise<gfx::Image> promise,