title: what are the differences between `event.sender.send` and `webContents.send` in electron?
body: <p>As this code snippet showed, both method can send event to the render process. I am wondering what are the differences between line A and line B in the code?</p>
  
  <pre><code>ipcMain.on('async', (event, arg) =&gt; {          
      console.log(arg);      
      event.sender.send('async-reply', 2); // line A
  });
  
  
  ipcMain.on('sync', (event, arg) =&gt; {          
      console.log(arg);
      event.returnValue = 4;
      mainWindow.webContents.send('ping', 5); //line B
  });
  </code></pre>