Allow resizing tracks in the top part of the interface

I often need it when looking at @fqueze's CI profile that oftentime are about resources. e.g. on 8 core machine (typical), we only see a few pixels, it's hard to see if we're constantly pegging a core (a flat bar at 16%) or if it's a multi-thread workload (usually more variable).

Same for CPU usage per category etc.

I can always run the cursor against the entire width to get a feel, but I'd rather make it full height and eyeball it.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-1269)