Only delete old frames when a new main frame navigation commits

chrome/test/data/extensions/api_test/webnavigation/test_getFrame.html

 <script>
 var URL = chrome.extension.getURL("getFrame/a.html");
 var tabId = -1;
 var processId = -1;
 
 chrome.test.runTests([
   function testGetFrame() {
     chrome.tabs.create({"url": "about:blank"}, function(tab) {
       tabId = tab.id;
       var done = chrome.test.listenForever(
-        chrome.webNavigation.onBeforeNavigate,
+        chrome.webNavigation.onCommitted,
         function (details) {
           if (details.tabId != tabId)
             return;
           if (details.url != URL)
             return;
           processId = details.processId;
           chrome.webNavigation.getFrame(
               {tabId: tabId, frameId: 0, processId: processId},
               function(details) {
             chrome.test.assertEq({errorOccurred: false, url: URL},
@@ skipping 22 matching lines @@
             [{errorOccurred: false,
               frameId: 0,
               processId: processId,
               url: URL}],
              details);
         chrome.test.succeed();
     });
   }
 ]);
 </script>