I was able to get this test to fail on the Windows port because the second frame begins loading the data URL before the first frame was done. On the Mac it appears that the second frame doesn't get to load in time, and the render tree is dumped prematurely.
I'm going to add a waitUntilDone() call to this test, so it always waits for the second frame to finish loading.
Created attachment 15169 [details]
Comment on attachment 15169 [details]
+ f.location = 'data:text/html,%3Cbody onload=%22layoutTestController.notifyDone();%22%3Efoo%3C/body%3E';
I think it would be nicer if you checked for layoutTestController there too.
Created attachment 15171 [details]
Same as first patch, but with mitz's suggestion.
Comment on attachment 15171 [details]
Landed in r23760.