It’s Adblock.
Even being disabled for the given site it still produces the error.
I was just about to reply with a similar thought. It looks like they are intentionally injecting code into about:blank. They even filed a Chrome bug/feature request for it. There’s basically nothing we can do about it on our end because they are injected code into all iFrames, even sandboxed ones. I would argue this is a bug in adblock.
Just to close the loop on this, Adblock says this is a Chrome bug and they opened the following issue for it: https://code.google.com/p/chromium/issues/detail?id=472101