DOMTimeStamp was renamed EpochTimeStamp. There is no observable behavioral change - it's just a name change. The HR-Time spec discourages the use of EpochTimeStamp, as it's a legacy feature.
Relevant WebIDL discussions/issue:
Which lead to:
Created attachment 440793 [details]
*** Bug 231498 has been marked as a duplicate of this bug. ***
Created attachment 440796 [details]
Committed r283912 (242783@main): <https://commits.webkit.org/242783@main>
All reviewed patches have been landed. Closing bug and clearing flags on attachment 440796 [details].