internal/experiment: re-add the new homepage experiment
internal/experiment: re-add the new homepage experiment

The change re-adds the new-homepage experiment after it
was removed in a bad merge in CL 241259.

Change-Id: If51d4b1d27abcf683baba0c8e71fcb0a11b3430b
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/245060
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite