Update expiry of ServceWorker LoadTiming histograms

These are still useful.

Bug: 1251250,1251251
Change-Id: I6e9d3afa6fdbf793578d2c29f8fa835946667281
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3171872


Commit-Queue: Ben Kelly <wanderview@chromium.org>
Reviewed-by: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/main@{#923361}