In the credits section of Chromium and Chrome (chrome://credits/) you can find XUL Runner SDK.
I've done some search in the chromium website and didn't find any sign of it nor for what is it used.
I was trying to decide between using something like chromiumembedded, Awesomium, WebKit, XUL Runner, libRocket, etc. so that finding is confusing.
So my question is: Why is XUL Runner SDK in chrome://credits/ ?
You should search the source code to check. The only actual use of the XULRunner SDK appears to be in mock_ie_event_sink_test.h where an accessibility-related file is being included. Constants from that file (like IA2_EVENT_DOCUMENT_LOAD_COMPLETE) are then being used in mock_ie_event_sink_test.cc - a unit test file. So the dependency on XULRunner doesn't seem to apply to any code actually shipping with Google Chrome.