Issue 9483 suggests several approaches to correlating logs from
machines in different time zones. This approach is the simplest and
really should be sufficient: provide a way to clamp the time stamps
to UTC.

Fixes #9483.

Change-Id: If540b991d758c4d845a719779f8255ece7c452e7
Reviewed-on: https://go-review.googlesource.com/8761Reviewed-by: Ian Lance Taylor <iant@golang.org>