Make it easier to fully expand the timeline vertically

[arai-a]

This is a part of steps explained in #5736

When comparing profiles, I always fully expand the timeline vertically, to make the screenshots and visual progress visible.
Currently, I need to grab the splitter and move it down until the timeline fully expands.
Given the splitter is vertically narrow, this takes some time, especially when I'm comparing ~100 profiles (this is necessary for investigating browsertime scores).
Also given that the height of the timeline differs between each suite or profile (depending on the number of content processes recorded),
I sometimes fail to fully expand, and also it needs extra care to verify it fully expands before stop dragging.

It would be nice if this can be done by just clicking a button, or something along that line.

┆Issue is synchronized with this Jira Task