Consider positioning the User Timing row as fixed on Stack Chart

When scrolling down in the stack chart UI it would be nice to have an indication of where the user timings are. Right now they are visible when scrolled to the top of the view, like on this profile https://share.firefox.dev/44Lstzp:


But when I scroll down into the stack I don't have a reminder where they are:


It might be hard to get the UX and implementation right here, so I'm just filing this as a suggestion to think about. For example, perhaps the timing markers should act sort of like position:fixed and be on top of anything that gets scrolled below it - if we did that maybe the color should be different to help distinguish from yellow stacks below it. Or there could be some indication in the bar above it, or fixed to the bottom like where the cookie notice is. FWIW in my case I don't care as much about the names of the markers, only where they are (i.e. whether the stack is happening inside of a range).

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-640)