Report TTFB after a bfcache restore

[philipwalton]

Now that CrUX is planning to move TTFB out of experimental and use a bfcache-aware version, this library should match. TTFB is the only metric collected by this library that doesn't have a bfcache aware version, so this change will be good for consistency as well.

However, since TTFB helps developers diagnose network or back-end issues, we should address #214 before we make this change, so developers can properly filter out these cases when needed.