Previously, we weren't recording most (all?) of the Next.js measurements like `Next.js-hydration` in Concurrent Mode. This was mainly because the new API doesn't accept a callback.
Instead of special casing this, I've refactored it so that the measurements are just recorded when Root first flushes (via `useLayoutEffect`), which should be more or less the same timing for the old API.
Concurrent Mode is a little trickier for two reasons:
1. Flushes might be (slightly) delayed due to time-slicing and prioritization
2. Selective hydration might skew measurements in cases where full hydration is aborted
I don't have a good answer for those yet, so they'll need to be addressed when the time comes.
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