Polish the UI for size profiles enough to be comfortable enabling it by default on the Firefox JSON Viewer

Known issues:
- [x] the interval in the profile info panel is shown with a time unit.
- [x] the stack chart tooltips show time units (but the flame graph tooltips show correct size units!)
- [ ] the CPU percentage line in the timeline tooltip doesn't make much sense.
- [ ] the 'Call Tree' panel name doesn't make sense for a file size (and the 'Stack Chart' name might not be great either).

More generally applicable issues that were mentioned in the JSON size profiler [review](https://phabricator.services.mozilla.com/D270623#9388796):
- [x] The track selector is still there for profiles with a single track
- [ ] The empty 'Marker Chart' and 'Marker Table' panels should be hidden for profiles without markers.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-939)