Check that callbackAdded callbacks are only called once.

chrome/renderer/resources/extensions/apitest.js

Index: chrome/renderer/resources/extensions/apitest.js
diff --git a/chrome/renderer/resources/extensions/apitest.js b/chrome/renderer/resources/extensions/apitest.js
index 6d8ea0ebb5ad363350e87a5ec7c6f16fd102734e..16bc078aa1a991533736a0929b3544b36a97c603 100644
--- a/chrome/renderer/resources/extensions/apitest.js
+++ b/chrome/renderer/resources/extensions/apitest.js
@@ -45,7 +45,11 @@
   chrome.test.callbackAdded = function() {
     pendingCallbacks++;
 
+    var called = false;
     return function() {
+      chrome.test.assertFalse(called, 'callback has already been run');
+      called = true;
+
       pendingCallbacks--;
       if (pendingCallbacks == 0) {
         chrome.test.succeed();