Replace prefetch logic (#251) is a 2% regression

A/B testing indicates this change caused a 2% score change in Safari.
This seems surprising since this was intended to be a purely harness
change. Reverting for now to confirm and to investigate the cause.

This also reverts "Add resource loading bar (#246)" and "Improve
Benchmark constructor and instance variables (#255)" as they blocked a
clean revert.

For posterity, this is tracked internally as rdar://problem/168103740

Author: kmiller68