As reported in bug 10958, fast/js/window-properties.html will fail when run with all the other layout tests, but running it by itself or running all of fast/js will show it passing. This is causing the failures on the buildbot.
*** Bug 10926 has been marked as a duplicate of this bug. ***
The failure seems to be caused by DumpRenderTree opening every test in the same WebView, thereby building up a session history. We need tests to be able to specify that they need a clean session.
r16720 fixes the checked-in results to match what you get when you do a full LayoutTests run. However, we still need a way for tests to specify they need a clean history.
layoutTestController.clearBackForwardList got added a long time ago (http://trac.webkit.org/changeset/25515, if not earlier).