Consider an immediate redirect as machine-initiated and slow one as...

chrome/test/data/History/history_length_test_page_1.html

Index: chrome/test/data/History/history_length_test_page_1.html
===================================================================
--- chrome/test/data/History/history_length_test_page_1.html	(revision 17847)
+++ chrome/test/data/History/history_length_test_page_1.html	(working copy)
@@ -13,17 +13,18 @@
 
 function onLoad() {
   if (readCookie(navigate_backward_cookie) != null) {
-    setTimeout(OnNavigateBackward, 100);
+    setTimeout(OnNavigateBackward, 50);
     return true;
   }
-  setTimeout(OnInitialLoad, 100);
+  setTimeout(OnInitialLoad, 50);
   return true;
 }
 
 function OnNavigateBackward() {
   if (window.history.length != 2) {
     onFailure("History_Length_Test_3", 1,
-              "History length mismatch on navigate backward at page 1");
+              "History length mismatch on navigate backward at page 1: " +
+              window.history.length);
     return false;
   }
   // Navigate forward from this point on.
@@ -35,7 +36,8 @@
 function OnInitialLoad() {
   if (window.history.length != 2) {
     onFailure("History_Length_Test_1", 1,
-              "History length mismatch on initial load at page 1");
+              "History length mismatch on initial load at page 1: " +
+              window.history.length);
     return false;
   }
   onSuccess("History_Length_Test_1", 1);