[DevTools] Show "Initial Paint" in the breadcrumbs when root is selected

[sebmarkbage]

We selected the root. This means that we're currently viewing the Transition that rendered the whole screen. In laymans terms this is really "Initial Paint". Once we add subtree selection, then the equivalent should be called "Transition" since in that case it's really about a Transition within the page. So if you've selected an Activity tree this should be called "Transition".

Once we add the environment support to the timeline. The first entry on the timeline should also be called "Initial Paint" when you haven't selected an Activity and "Transition" when you have.

Technically they're both meant to be "Transition" but nobody thinks of initial load as a "Transition" from the previous MPA page.

[sebmarkbage]

I guess I could make this clickable just like the other one. It's kind of weird, since it should always be selected already. It's not quite true today due to the annoying case where you can jump to a non-Suspense.