window.MutationRecord is undefined in Blink/WebKit but it is exposed in Gecko. The spec at http://www.w3.org/TR/domcore/#mutationrecord does not have a [NoInterfaceObject] so it means that the interface should be exposed on Window.
And here is Adam's fix from Blink. Should be straight forward to apply to WebKit https://codereview.chromium.org/13861028/
Created attachment 197659 [details] Patch
Comment on attachment 197659 [details] Patch No feature conditional?
Comment on attachment 197659 [details] Patch Clearing flags on attachment: 197659 Committed r148236: <http://trac.webkit.org/changeset/148236>
All reviewed patches have been landed. Closing bug.