In Marker Table, "FileIO" marker descriptions don't show the marker name

Since the Gecko Profiler can now record off-main thread markers, the marker name can now be something other than just "FileIO" (e.g.: "FileIO (unregistered thread)"), so it would be useful to show it in the Marker Table "Description" and not just in the details panel.

E.g.: https://share.firefox.dev/36SBedo , see marker at 0.766s.

(I'm guessing it's hidden because it used to always be "FileIO" like the marker type so it looked redundant?)

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-270)