Display vsync markers in the marker timeline at the top, as vertical lines across all threads

We now have `VsyncTimestamp` markers displayed in the marker chart, for example in this profile: https://perfht.ml/2EdNp9K

However, it's still hard to track vsync timing across threads. It would be more useful to also have vertical lines in the thread timeline at the top, across all threads.

I'm not sure how exactly this should look. We already have faint grey grid lines up there; adding more lines may make things look really busy. We probably also need a zoom threshold; if you're zoomed out enough that each line would only be 3 pixels or less apart, we should probably hide the lines completely.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-73)