Add interoperable TTFB to measure Early Hints consistently

[tunetheweb]

Fixes #565

This will revert Chrome back to reporting TTFB as the Early Hints first bytes (same as other browsers and tools).

[tunetheweb]

@philipwalton WDYT about this one?

I know we agreed not to add a documentStart timeline (which has been removed from this PR) since that's easily getable by looking at finalHeadersResponseStart (about to be launched in Chrome 133), so we could not handle Chrome 115-133 differently since that's due to launch, but normally we try to support things for two years before we consider them no longer required to work around, so should we still merge this?

At the very least I think the Early Hints test is worth saving, but can move that to a separate PR. But it would need to wait for Chrome 133 to land if we don't merge this.