title: chrome extension `sendResponse` does not work
body: <p>I am writing a chrome extension but the <code>sendResponse</code> method does not work.</p>
  <pre><code>chrome.runtime.onMessage.addListener(
      function(request, sender, sendResponse) {
  
              if(!request.method){
                      return false;
              }
  
              if(request.method=='postList' &amp;&amp; request.post_list){
                  // alert(1);                                                                                                                                                                            
                      a_facebook_api.getPostFromDB(request.post_list, function(data){
                              alert(data);                                                                                                                                                              
                              sendResponse(data);                                                                                                                                                       
                            
                      });
  
              } else if(request.method=='postAdd' &amp;&amp; request.post_data){
                      a_facebook_api.addPostToDB(request.post_data, function(data){
  
                              sendResponse(data);
                      });
  
      }
  
              return true;
  
  }
  );
  
   chrome.runtime.sendMessage({method: &quot;postList&quot;,post_list: post_list}, function(response) {
  
           alert(response);
                                                                                                                                                             
              });
  </code></pre>
  <p>the function <code>alert(data)</code> works. It gives me proper data with JSON format. However, the <code>alert(response)</code> does not show any message. Can anyone give me some ideas why it isn't working?</p>
  <p>Thank you in advance!</p>