GCMajor and other "nonexhaustive range" markers should be more clearly distinguished visually

I thought I had an issue for this, but perhaps it was just in my head. It would fit in with all the other disorganized junk in there.

It seems to be a repeated source of confusion that the GCMajor marker is not a solid block of time where we are only GCing. The latest example is https://bugzilla.mozilla.org/show_bug.cgi?id=1673457 (though that isn't the primary confusion there.) It would be helpful if there were some sort of visual distinction between markers that are "this is the one thing happening during this time period" (even if other markers are nested within) vs "this is a time range where X is going on, but plenty of other things could be happening during the same interval".

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-333)