Per WebKit's update, https://github.com/WebKit/WebKit/pull/71850, I
think this makes more sense to make the computed offset be unresolved,
instead of +/-Infinity, given that we cannot find a specified point in
the timeline with zero animation attachment range.
This aligns the behavior of WebKit and Chromium.
Differential Revision: https://phabricator.services.mozilla.com/D324772